Descartes defined matter in terms of extension. It could just as well be defined in terms of time, for both the one and the other are external conditions of its existence in spatial and temporal relations. Thus both become also the necessary conditions of measurement for all thinking minds, which are themselves limited by place and time, but they never become the essence of matter. Spinoza struggled for a long time against this Cartesian explanation, probably because he felt that something about it was not clear. He was not satisfied with his teacher's sharp distinction between matter and spirit, but, since he lacked a unifying intermediate conception, what could he do? Unfortunately then, in his Ethics he still took matter for extension, that is space, and set it up beside thought, an entirely different kind of thing. Now he was indeed on the way to a very intricate confusion.

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.

In the history of philosophy we meet with Substance as the principle of Spinoza's system. On the import and value of this much-praised and no-less decried philosophy there has been great misunderstanding and a deal of talking since the days of Spinoza. The atheism, and as a further charge, the pantheism of the system has formed the commonest ground of accusation. These cries arise because of Spinoza's conception of God as substance, and substance only. What we are to think of this charge follows, in the first instance, from the place which substance takes in the system of the logical idea. Though an essential stage in the evolution of the idea, substance is not the same with absolute idea, but the idea under the still limited form of necessity.

Do not be mistaken about the word “Substance.” Spinoza took it in its purest meaning, and had to take it in that way if he wanted to proceed geometrically and set down a primitive notion as a basis. What is Substance but a thing which is self-dependent, which has the cause of its existence in itself? I wish that this pure meaning of the word could have been introduced into our philosophy. In the strictest sense, nothing in the world is a Substance, because everything depends on everything else, and finally on God, who therefore is the highest and only Substance. This geometrical conception could not have become generally adopted in a philosophy which must preserve its popular character, for we, in all our dependence yet consider ourselves independent, and in a certain sense, as we shall soon see, we may so consider ourselves.

By using the term 'thinking substance' in such sharp contrast to 'extended substance' [Descartes] was clearly implying that the various distinct forms appearing in thought do not have their existence in such an order of extension and separation (i.e., some kind of space), but rather in a different order, in which extension and separations have no fundamental significance.

If there is a single initial error at the root of all the difficulties philosophy is involved in, it can only be the one Descartes committed when he decreed, a priori, that the method of one of the sciences of reality was valid for the whole of reality.

Descartes, a genius made to blaze new paths and to go astray in them, supposed with some other philosophers that God is the only efficient cause of motion, and that every instant He communicates motion to all bodies. But this opinion is but an hypothesis which he tried to adjust to the light of faith; and in so doing he was no longer attempting to speak as a philosopher or to philosophers. Above all he was not addressing those who can be convinced only by the force of evidence.

The latter [Descartes] had sharply divided nature into mind and matter, i.e., thinking and extended substance, and had also placed God and the world in complete opposition to each other; Spinoza also, so long as he was a Cartesian, taught all that in his “Cogitatis Metaphysicis,” c. 12, i. I., 1665. Only in his later years did he see the fundamental falseness of that double dualism; and accordingly his own philosophy principally consists of the indirect abolition of these two antitheses. Yet partly to avoid injuring his teacher, partly in order to be less offensive, he gave it a positive appearance by means of a strictly dogmatic form, although its content is chiefly negative. His identification of the world with God has also this negative significance alone. For to call the world God is not to explain it: it remains a riddle under the one name as under the other. But these two negative truths had value for their age, as for every age in which there still are conscious or unconscious Cartesians.

Hence it inevitably follows logically, as Engels said, ‘that matter remains eternally the same in all its transformations, that none of its attributes can ever be lost, and therefore, also, that with the same iron necessity that it will exterminate on the earth its highest creation, the thinking mind, it must somewhere else and at another time again produce it.’ That was Spinoza's standpoint, a circumstance that seemingly gave Engels grounds for replying categorically and unambiguously to Plekhanov when he asked: ‘So in your opinion old Spinoza was right in saying that thought and extension were nothing but two attributes of one and the same substance?’ "Of course," answered Engels, "old Spinoza was quite right".’ Spinoza's definition means the following: in man, as in any other possible thinking creature, the same matter thinks as in other cases (other modi) only ‘extends’ in the form of stones or any other ‘unthinking body’; that thought in fact cannot be separated from world matter and counterposed to it itself as a special, incorporeal ‘soul’, and it (thought) is matter’s own perfection. That is how Herder and Goethe, La Mettrie and Diderot, Marx and Plekhanov (all great ‘Spinozists’) and even the young Schelling, understood Spinoza. [...] Spinoza said more than once that it was impermissible to represent thought as attribute in the image and likeness of human thought; it was only the universal property of substance that was the basis of any ‘finite thought’, including human thought, but in no case was it identical with it. To represent thought in general in the image and likeness of existing human thought, of its modus, or ‘particular case’, meant simply to represent it incorrectly, in ‘an incomplete way’, by a ‘model’, so to say, of its far from most perfected image (although the most perfected known to us).

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.

The brilliance of the solution of the problem of the relation of thinking to the world of bodies in space outside thought (i.e. outside the head of man), which Spinoza formulated in the form of the thesis that thought and extension are not two substances, but only two attributes of one and the same substance, can hardly be exaggerated. This solution immediately rejected every possible kind of interpretation and investigation of thought by the logic of spiritualist and dualist constructions, so making it possible to find a real way out both from the blind alley of the dualism of mind and body and from the specific blind alley of Hegelianism. It is not fortuitous that Spinoza’s profound idea only first found true appreciation by the dialectical materialists Marx and Engels. Even Hegel found it a hard nut to crack. In fact, on the decisive point, he returned again to the position of Descartes, to the thesis that pure thought is the active cause of all the changes occurring in the ‘thinking body of man’, i.e. in the matter of the brain and sense organs, in language, in actions and their results, including in that the instruments of labour and historical events.

After Kepler’s bold and fruitful efforts to advance natural philosophy by the help of geometry, there should have appeared any philosopher and particularly a geometer, namely Descartes, who should leave this one narrow path and try to investigate the causes of things logically, or rather, sophistically. What is to be said of him who while certainly learned in geometry would build his cosmic system (which he valued so highly and of which he boasted so grandiloquently) from vortices, without previously examining whether bodies carried around by a vortex at different distances from the centre would have periodic times whose squares were as the cubes of the distances from the centre? But he was intoxicated by easier and less composite laws, and, not applying his geometric ability in the slightest, fell into errors from which we were at length liberated by the aid of geometers.

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.

Taken as a whole, this constitutes the Idea of Spinoza, and it is just what pure being was to the Eleatics (Vol. 1. pp. 244, 252). This Idea of Spinoza's we must allow to be in the main true and well-grounded; absolute substance is the truth, but it is not the whole truth; in order to be this it must also be thought of as in itself active and living, and by that very means it must determine itself as mind. But substance with Spinoza is only the universal and consequently the abstract determination of mind; it may undoubtedly be said that this thought is the foundation of all true views — not, however, as their absolutely fixed and permanent basis, but as the abstract unity which mind is in itself. It is therefore worthy of note that thought must begin by placing itself at the standpoint of Spinozism; to be a follower of Spinoza is the essential commencement of all Philosophy. For as we saw above (Vol. I. p. 144), when man begins to philosophize, the soul must commence by bathing in this ether of the One Substance, in which all that man has held as true has disappeared; this negation of all that is particular, to which every philosopher must have come, is the liberation of the mind and its absolute foundation. The difference between our standpoint and that of the Eleatic philosophy is only this, that through the agency of Christianity concrete individuality is in the modern world present throughout in spirit. But in spite of the infinite demands on the part of the concrete, substance with Spinoza is not yet determined as in itself concrete. As the concrete is thus not present in the content of substance, it is therefore to be found within reflecting thought alone, and it is only from the endless oppositions of this last that the required unity emerges. Of substance as such there is nothing more to be said; all that we can do is to speak of the different ways in which Philosophy has dealt with it, and the opposites which in it are abrogated. The difference depends on the nature of the opposites which are held to be abrogated in substance. Spinoza is far from having proved this unity as convincingly as was done by the ancients; but what constitutes the grandeur of Spinoza's manner of thought is that he is able to renounce all that is determinate and particular, and restrict himself to the One, giving heed to this alone.