"Let us imagine," says Spinoza in his fifteenth letter, "a little worm living in blood which has vision enough to discern the particles of blood, lymph, &c., and reason enough to observe how one particle is repelled by another with which it comes into contact, or communicates a part of its motion to it. Such a worm would live in the blood as we do in this part of the universe, and would regard each particle of it, not as a part, but as a whole, nor could it know how all the parts are influenced by the universal nature of the blood, and are obliged to accommodate themselves to each other as is required by that nature, so that they co-operate together according to a fixed law. For if we suppose that there are no causes outside of the blood which could communicate new motions to it, and no space beyond the blood, nor any other bodies to which its particles could transfer their motion, it is certain that the blood as a whole would always maintain its present state, and its particles would suffer no other variations than those which may be inferred from the given relation of the motion of the blood to lymph, chyle, &c. And thus in that case the blood would require to be considered always as a whole and not as a part. But since there are many other causes which influence the laws of the nature of blood, and are in turn influenced thereby, other motions and other variations must arise in the blood which are not due to the proportion of motion in its constituents but also to the relation between that motion and external causes. And therefore we cannot consider the blood as a whole, but only as a part of a greater whole."
"Now we can think, and indeed ought to think, of all natural bodies in the same manner in which we have thought of this blood, for all bodies are surrounded by other bodies, and reciprocally determine and are determined by them, to exist and operate in a fixed and definite way, so as to preserve the same ratio of motion and rest in the whole universe. Hence it follows that every body, in so far as it exists under a certain definite modification, ought to be considered as merely a part of the whole universe which agrees with its whole, and thereby is in intimate union with all the other parts; and since the nature of the universe is not limited like that of the blood, but absolutely infinite, it is clear that by this nature, with its infinite powers, the parts are modified in an infinite number of ways, and compelled to pass through an infinity of variations. Moreover, when I think of the universe as a substance, I conceive of a still closer union of each part with the whole; for, as I have elsewhere shown, it is the nature of substance to be infinite, and therefore every single part belongs to the nature of the corporeal substance, so that apart therefrom it neither can exist nor be conceived. And as to the human mind, I think of it also as of part of nature, for I think of nature as having in it an infinite power of thinking, which, as infinite, contains in itself the idea of all nature, and whose thoughts run parallel with all existence."