But then with me the horrid doubt always arises whether the convictions of man's mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would any one trust in the convictions of a monkey's mind, if there are any convictions in such a mind?
[To William Graham 3 July 1881]

Man bears in his bodily structure clear traces of his descent from some lower form; but it may be urged that, as man differs so greatly in his mental power from all other animals, there must be some error in this conclusion. No doubt the difference in this respect is enormous, even if we compare the mind of one of the lowest savages, who has no words to express any number higher than four, and who uses no abstract terms for the commonest objects or affections, with that of the most highly organised ape.

The mind of man is capable of anything.

Even the heavens are open to the mind of Man. Man alone of all the animals has traced the pathways of the rising and the setting of the stars. Man has measured out the days and months and years. Man alone has understood and can predict the eclipses of the sun and moon for all future time, when they will occur, and whether they will be partial or total. When the mind contemplates these phenomena, it learns also knowledge of the gods. So religion is born, and with it goodness and all the virtues which make up the good life, a life which reflects the divine life. We need to be inferior to the gods in nothing except our mortality, which need in no way hinder us from living well. In explaining these things, I think that I have shown clearly enough how much superior is human nature to that of all the other animals. From which we must infer that such a shape and arrangement of our limbs and such a power of intelligence cannot have been the work of chance alone.

Man’s mind is so formed that it is far more susceptible to falsehood than to truth.

Doth any man doubt, that if there were taken out of men's minds vain opinions, flattering hopes, false valuations, imaginations as one would, and the like, but it would leave the minds of a number of men poor shrunken things, full of melancholy and indisposition, and unpleasing to themselves?

Personally, I do not believe the human mind has any limits but those we impose ourselves.

The ancient lizard mind lies below the mammalian mind, which lies below a primate mind, which is modified by a mind adapted to language, and since these layers have developed in response to differing evolutionary pressures, they often do not function efficiently together. Human civility tries to control ape dominance, human rationality tries to control mammalian sexuality, human social conscience tries to ameliorate reptilian greed, never with total success.

What is troubling us is the tendency to believe that the mind is like a little man within.

Man is a credulous animal, and must believe something; in the absence of good grounds for belief, he will be satisfied with bad ones.

Not Chaos, not The darkest pit of lowest Erebus, Nor aught of blinder vacancy, scooped out By help of dreams — can breed such fear and awe As fall upon us often when we look Into our Minds, into the Mind of Man.

Man is the Reasoning Animal. Such is the claim. I think it is open to dispute.

Normally man’s mind is composed only of a consciousness of his immediate needs, which is to say that this consciousness at any moment can be defined as ‘’his awareness of his own power to satisfy those needs.’’ He thinks in terms of what he intends to do in half an hour’s time, a day’s time, a month’s time an no more. He never asks himself: what are the ‘’limits’’ of my powers? In a sense, he is like a man who has a fortune is the bank, who never asks himself, How much money have I got, but only, Have I enough for a pound of cheese, a new tie, etc.

It has been said that man is a rational animal; while this formulation is insufficient and ill-sounding, it nonetheless points to an undeniable truth, though in an elliptical fashion, for the rational faculty actually serves to underscore the transcendence of man in relation to the animal. Man is rational because he possesses the Intellect, which by definition has a capacity for the absolute and therefore a sense of the relative; and he possesses the Intellect because he is made "in the image of God", which, moreover, he demonstrates physically by his corporeal form and his cranial form, as well as by his vertical posture, then by language and his productive capacity. Man is a theophany in his form as much as in his faculties.