The second point to be considered is the method adopted by Spinoza for setting forth his philosophy; it is the demonstrative method of geometry as employed by Euclid, in which we find definitions, explanations, axioms, and theorems. Even Descartes made it his starting-point that philosophic propositions must be mathematically handled and proved, that they must have the very same evidence as mathematics. The mathematical method is considered superior to all others, on account of the nature of its evidence; and it is natural that independent knowledge in its re-awakening lighted first upon this form, of which it saw so brilliant an example. The mathematical method is, however, ill-adapted for speculative content, and finds its proper place only in the finite sciences of the understanding. In modern times Jacobi has asserted (Werke, Vol. IV. Section I. pp. 217-223) that all demonstration, all scientific knowledge leads back to Spinozism, which alone is a logical method of thought; and because it must lead thither, it is really of no service whatever, but immediate knowledge is what we must depend on. It may be conceded to Jacobi that the method of demonstration leads to Spinozism, if we understand thereby merely the method of knowledge belonging to the understanding. But the fact is that Spinoza is made a testing-point in modern philosophy, so that it may really be said: You are either a Spinozist or not a philosopher at all. This being so, the mathematical and demonstrative method of Spinoza would seem to be only a defect in the external form; but it is the fundamental defect of the whole position. In this method the nature of philosophic knowledge and the object thereof, are entirely misconceived, for mathematical knowledge and method are merely formal in character and consequently altogether unsuited for philosophy. Mathematical knowledge exhibits its proof on the existent object as such, not on the object as conceived; the Notion is lacking throughout; the content of Philosophy, however, is simply the Notion and that which is comprehended by the Notion. Therefore this Notion as the knowledge of the essence is simply one assumed, which falls within the philosophic subject; and this is what represents itself to be the method peculiar to Spinoza's philosophy.

The second point to be considered is the method adopted by Spinoza for setting forth his philosophy; it is the demonstrative method of geometry as employed by Euclid, in which we find definitions, explanations, axioms, and theorems. Even Descartes made it his starting-point that philosophic propositions must be mathematically handled and proved, that they must have the very same evidence as mathematics. The mathematical method is considered superior to all others, on account of the nature of its evidence; and it is natural that independent knowledge in its re-awakening lighted first upon this form, of which it saw so brilliant an example. The mathematical method is, however, ill-adapted for speculative content, and finds its proper place only in the finite sciences of the understanding. In modern times Jacobi has asserted (Werke, Vol. IV. Section I. pp. 217-223) that all demonstration, all scientific knowledge leads back to Spinozism, which alone is a logical method of thought; and because it must lead thither, it is really of no service whatever, but immediate knowledge is what we must depend on. It may be conceded to Jacobi that the method of demonstration leads to Spinozism, if we understand thereby merely the method of knowledge belonging to the understanding. But the fact is that Spinoza is made a testing-point in modern philosophy, so that it may really be said: You are either a Spinozist or not a philosopher at all. This being so, the mathematical and demonstrative method of Spinoza would seem to be only a defect in the external form; but it is the fundamental defect of the whole position. In this method the nature of philosophic knowledge and the object thereof, are entirely misconceived, for mathematical knowledge and method are merely formal in character and consequently altogether unsuited for philosophy. Mathematical knowledge exhibits its proof on the existent object as such, not on the object as conceived; the Notion is lacking throughout; the content of Philosophy, however, is simply the Notion and that which is comprehended by the Notion. Therefore this Notion as the knowledge of the essence is simply one assumed, which falls within the philosophic subject; and this is what represents itself to be the method peculiar to Spinoza's philosophy.

If Spinoza had in fact tried to construct his philosophical system by the method that our contemporary positivism would have recommended to him, it is not difficult to imagine what he would have produced as a ‘system’. He would only have brought together the purely mechanical and religious, mystical ‘general ideas’ that were guiding all (or almost all) naturalists in his day. Spinoza understood very clearly that religious, theological mysticism was the inevitable complement of a purely mechanistic (geometrical, mathematical) world outlook, i.e. the point of view that considers the sole ‘objective’ properties of the real world to be only the spatial, geometrical forms and relations of bodies. His greatness was that he did not plod along behind contemporaneous natural science, i.e. behind the one-sided, mechanistic thinking of the coryphaei of the science of the day, but subjected this way of thinking to well substantiated criticism from the angle of the specific concepts of philosophy as a special science. This feature of Spinoza’s thinking was brought out clearly and explicitly by Friedrich Engels: ‘It is to the highest credit of the philosophy of the time that it did not let itself be led astray by the restricted state of contemporary natural knowledge, and that from Spinoza right to the great French materialists it insisted on explaining the world from the world itself and left the justification in detail to the natural science of the future.’ [Dialectics of Nature] That is why Spinoza has come down in the history of science as an equal contributor to its progress with Galileo and Newton, and not as their epigone, repeating after them the general ideas that could be drawn from their work. He investigated reality himself from the special, philosophical angle, and did not generalise the results and ready-made findings of other people’s investigation, did not bring together the general ideas of the science of his day and the methods of investigation characteristic of it, or the methodology and logic of his contemporary science. He understood that that way led philosophy up a blind alley, and condemned it to the role of the wagon train bringing up in the rear of the attacking army the latter’s own ‘general ideas and methods’, including all the illusions and prejudices incorporated in them. That is why he also developed ‘general ideas and methods of thought’ to which the natural science of the day had not yet risen, and armed future science with them, which recognised his greatness three centuries later through the pen of Albert Einstein, who wrote that he would have liked ‘old Spinoza’ as the umpire in his dispute with Niels Bohr on the fundamental problems of quantum mechanics rather than Carnap or Bertrand Russell, who were contending for the role of the ‘philosopher of modern science’ and spoke disdainfully of Spinoza’s philosophy as an ‘outmoded’ point of view ‘which neither science nor philosophy can nowadays accept’. Spinoza's understanding of thinking as the activity of that same nature to which extension also belonged is an axiom of the true modern philosophy of our century, to which true science is turning more and more confidently and consciously in our day (despite all the attempts to discredit it) as the point of view of true materialism.

As regards the philosophy of Spinoza, it is very simple, and on the whole easy to comprehend; the difficulty which it presents is due partly to the limitations of the method in which Spinoza presents his thoughts, and partly to his narrow range of ideas, which causes him in an unsatisfactory way to pass over important points of view and cardinal questions. Spinoza's system is that of Descartes made objective in the form of absolute truth. The simple thought of Spinoza's idealism is this: The true is simply and solely the one substance, whose attributes are thought and extension or nature: and only this absolute unity is reality, it alone is God. It is, as with Descartes, the unity of thought and Being, or that which contains the Notion of its existence in itself. The Cartesian substance, as Idea, has certainly Being included in its Notion; but it is only Being as abstract, not as real Being or as extension (supra, p. 241). With Descartes corporeality and the thinking 'I' are altogether independent Beings; this independence of the two extremes is done away with in Spinozism by their becoming moments of the one absolute Being. This expression signifies that Being must be grasped as the unity of opposites; the chief consideration is not to let slip the opposition and set it aside, but to reconcile and resolve it. Since then it is thought and Being, and no longer the abstractions of the finite and infinite, or of limit and the unlimited, that form the opposition (supra, p. 161), Being is here more definitely regarded as extension; for in its abstraction it would be really only that return into itself, that simple equality with itself, which constitutes thought (supra, p. 229). The pure thought of Spinoza is therefore not the simple universal of Plato, for it has likewise come to know the absolute opposition of Notion and Being.

The shortcoming thus acknowledged to attach to the content turns out at the same time to be a shortcoming in respect of form. Spinoza puts substance at the head of his system, and defines it to be the unity of thought and extension, without demonstrating how he gets to this distinction, or how he traces it back to the unity of substance. The further treatment of the subject proceeds in what is called the mathematical method. Definitions and axioms are first laid down: after them comes a series of theorems, which are proved by an analytical reduction of them to these unproved postulates. Although the system of Spinoza, and that even by those who altogether reject its contents and results, is praised for the strict sequence of its method, such unqualified praise of the form is as little justified as an unqualified rejection of the content. The defect of the content is that the form is not known as immanent in it, and therefore only approaches it as an outer and subjective form. As intuitively accepted by Spinoza without a previous mediation by dialectic, Substance, as the universal negative power, is as it were a dark shapeless abyss which engulfs all definite content as radically null, and produces from itself nothing that has a positive subsistence of its own.

...Spinoza hit the nail on the head with his paradoxical proposition: God is an extended, that is, material being. He found, at least for his time, the true philosophical expression for the materialistic tendency of the modern era; he legitimated and sanctioned it: God himself is a materialist. Spinoza's philosophy was religion; he himself was an amazing man. Unlike so many others, Spinoza's materialism did not stand in contradiction to the notion of a non-material and anti-materialistic God who also quite consistently imposes on man the duty to give himself up only to anti-materialistic, heavenly tendencies and concerns, for God is nothing other than the archetypal and ideal image of man; what God is and how he is, is what man ought to be or wants to be, or at least hopes to be in the future. But only where theory does not belie practice, and practice theory, is there character, truth, and religion. Spinoza is the Moses of modern free-thinkers and materialists.

Spinoza founds Modern materialism in its highest form, determining the horizons of both Modern and contemporary philosophical speculation within an immanent and given philosophy of being and an atheism defined as the negation of every presupposed ordering of either the constitution of being or human behavior. However, even in its productive and living form, Spinozian metaphysics does not succeed in superseding the limits of a purely "spatial" (or Galilean-physical) conception of the world. It certainly pushes on this conception and tries to destroy its limits, but it does not reach a solution. Rather, it leaves unresolved the problem of the relationship between the spatial dimensions and the temporal, creative, and dynamic dimensions of being. The imagination, that spiritual faculty running throughout the Spinozian system, constitutes being in an order that is only allusively temporal. As such, the problem remains intact, in terms that are unresolved but pure and forceful: Being (before the invention of the dialectic) evades the tangle of dialectical materialism. In fact, the readings of Spinoza by socialist and Soviet authors have not enriched dialectical materialism but have, rather, only diminished the potentialities that Spinozian metaphysics offers for superseding the purely spatial and objectivistic dimension of materialism.

One of Spinoza's most important conclusions is that of the human being's necessity to overcome the contradiction between the finite and the infinite. Spinoza was able to express the very nature of the Judaeo-Christian way of thought and, at the same time, to remain outside it and even negate it. Both in the Jewish and the Christian traditions, God creates the world but is outside it. Spinoza, on the other hand, would not say that God creates the world, but that He produces it – in philosophical terms, He causes it. God, for Spinoza, is not outside the world and this view was the object of very harsh criticism by his contemporaries. The Jewish community in Holland even saw fit to excommunicate him. The God of Judeao-Christian thought, according to Spinoza, is an invention of man, who imagines that God thinks and acts as human beings do.

Spinoza’s metaphysics is, in many ways, an effort to tap into the underlying rationalist motivations of Descartes’s metaphysics and to follow through on these motivations more consistently than Descartes ever did. Employing the Cartesian notions of substance, attribute, and mode, and wielding strongly rationalist principles only hinted at in Descartes—such as the PSR and the Principle of the Identity of Indiscernibles—Spinoza is able to mount a powerful argument for substance monism, for the view that there is, fundamentally, only one thing in the world.

Spinoza believed that he had, through metaphysical inquiry, discovered important truths about God, nature and human beings, truths that led to principles of great consequence for our happiness and our emotional and physical flourishing. This, in fact, is what he called “true religion.” There is a lesson here: By enforcing conformity of belief and punishing deviations from dogma, religious authorities may end up depriving the devoted of the possibility of achieving in religion that which they most urgently seek.

It is true that God is necessity, or, as we may also put it, that he is the absolute Thing: he is however no less the absolute Person. That he is the absolute Person however is a point which the philosophy of Spinoza never reached: and on that side it falls short of the true notion of God which forms the content of religious consciousness in Christianity. Spinoza was by descent a Jew; and it is upon the whole the Oriental way of seeing things, according to which the nature of the finite world seems frail and transient, that has found its intellectual expression in his system. This Oriental view of the unity of substance certainly gives the basis for all real further development. Still it is not the final idea. It is marked by the absence of the principle of the Western world, the principle of individuality, which first appeared under a philosophic shape, contemporaneously with Spinoza, in the Monadology of Leibnitz.

Spinoza’s formula, “Deus=natura=substantia”, like all conclusions drawn from it in his Ethics and his earlier works, simply means that there is no God. This discovery of Spinoza’s became the starting point for modern philosophical thought. Foreword p. xxxix

Spinoza was the man I believed in always, as the alternative to Catholicism. And it is only in Spinoza's manner that I am a positivist at all. I believe in the real world, in the world of thought and extension, of psychology and physics. God or substance with Spinoza equals reality; and this reality, which may have countless forms, we find only in space and in (other men's) consciousness. I say in other men's, because Spinoza was too sane to care to discuss anything from the point of view of subjective idealism. When one prints a book to convince other people, one oughtn't to discuss in it whether they exist. But this is all by the way, although some other day I will discuss with you the question of Spinoza's hedonism.

Spinoza is a highly systematic thinker, but still I do not think I can offer a single key for all things Spinozistic. Personally, one thing which got me excited about Spinoza is his philosophical boldness, i.e., his willingness to pursue philosophical exploration as far as he can, making very little concessions to commonly accepted beliefs and norms. In terms of content, I take his attempt to conceive of God, nature, and ethics in a manner that is free from anthropomorphism and anthropocentric illusions as one of the deepest elements of his philosophical thinking. A closely related issue is his advocacy of actual infinity (an issue that has been mostly neglected in recent literature). Finally, the very attempt to do philosophy systematically (rather than rely on fragmented and disassociated intuitions) and transparently (laying bare the logical structure of his arguments) commands my respect, indeed admiration.