For the encouragement to pursue my speculations given me by yourself and the distinguished R. Boyle, I return you my best thanks. I proceed as far as my slender abilities will allow me, with full confidence in your aid and kindness. When you ask me my opinion on the question raised concerning our knowledge of the means, whereby each part of nature agrees with its whole, and the manner in which it is associated with the remaining parts, I presume you are asking for the reasons which induce us to believe, that each part of nature agrees with its whole, and is associated with the remaining parts. For as to the means whereby the parts are really associated, and each part agrees with its whole, I told you in my former letter that I am in ignorance. To answer such a question, we should have to know the whole of nature and its several parts. I will therefore endeavour to show the reason, which led me to make the statement; but I will premise that I do not attribute to nature either beauty or deformity, order or confusion. Only in relation to our imagination can things be called beautiful or deformed, ordered or confused.
By the association of parts, then, I merely mean that the laws or nature of one part adapt themselves to the laws or nature of another part, so as to cause the least possible in consistency. As to the whole and the parts, I mean that a given number of things are parts of a whole, in so far as the nature of each of them is adapted to the nature of the rest, so that they all, as far as possible, agree together. On the other hand, in so far as they do not agree, each of them forms, in our mind, a separate idea, and is to that extent considered as a whole, not as a part. For instance, when the parts of lymph, chyle, etc., combine, according to the proportion of the figure and size of each, so as to evidently unite, and form one fluid, the chyle, lymph, etc., considered under this aspect, are part of the blood; but, in so far as we consider the particles of lymph as differing in figure and size from the particles of chyle, we shall consider each of the two as a whole, not as a part.
Let us imagine, with your permission, a little worm, living in the blood, able to distinguish by sight the particles of blood, lymph, etc., and to reflect on the manner in which each particle, on meeting with another particle, either is repulsed, or communicates a portion of its own motion. This little worm would live in the blood, in the same way, as we live in a part of the universe, and would consider each particle of blood, not as a part, but as a whole. He would be unable to determine, how all the parts are modified by the general nature of blood, and are compelled by it to adapt themselves, so as to stand in a fixed relation to one another. For, if we imagine that there are no causes external to the blood, which could communicate fresh movements to it, nor any space beyond the blood, nor any bodies whereto the particles of blood could communicate their motion, it is certain that the blood would always remain in the same state, and its particles would undergo no modifications, save those which may be conceived as arising from the relations of motion existing between the lymph, the chyle, etc. The blood would then always have to be considered as a whole, not as a part. But, as there exist, as a matter of fact, very many causes which modify, in a given manner, the nature of the blood, and are, in turn, modified thereby, it follows that other motions and other relations arise in the blood, springing not from the mutual relations of its parts only, but from the mutual relations between the blood as a whole and external causes. Thus the blood comes to be regarded as a part, not as a whole. So much for the whole and the part.
All natural bodies can and ought to be considered in the same way as we have here considered the blood, for an bodies are surrounded by others, and are mutually determined to exist and operate in a fixed and definite proportion while the relations between motion and rest in the sum total of them, that is, in the whole universe, remain unchanged. Hence it follows that each body, in so far as it exists as modified in a particular manner, must be considered as a part of the whole universe, as agreeing with the whole, and associated with the remaining parts. As the nature of the universe is not limited, like the nature of blood, but is absolutely infinite, its parts are by this nature of infinite power infinitely modified, and compelled to undergo infinite variations. But, in respect to substance, I conceive that each part has a more close union with its whole. For, as I said in my first letter, (addressed to you while I was still at Rhijnsburg), substance being infinite in its nature, it follows, as I endeavoured to show, that each part belongs to the nature of substance, and, without it, can neither be nor be conceived.
You see, therefore, how and why I think that the human body is a part of nature. As regards the human mind, I believe that it also is a part of nature; for I maintain that there exists in nature an infinite power of thinking, which, in so far as it is infinite, contains subjectively the whole of nature, and its thoughts proceed in the same manner as nature--that is, in the sphere of ideas. Further, I take the human mind to be identical with this said power, not in so far as it is infinite, and perceives the whole of nature, but in so far as it is finite, and perceives only the human body; in this manner, I maintain that the human mind is a part of an infinite understanding.
But to explain, and accurately prove, all these and kindred questions, would take too long; and I do not think you expect as much of me at present. I am afraid that I may have mistaken your meaning, and given an answer to a different question from that which you asked. Please inform me on this point.
in your last letter, that I hinted that nearly all the Cartesian laws
of motion are false. What I said was, if I remember rightly, that Huyghens
thinks so; I myself do not impeach any of the laws except the sixth, concerning
which I think Huyghens is also in error. I asked you at the same time
to communicate to me the experiment made according to that hypothesis
in your Royal Society; as you have not replied, I infer that you are not
at liberty to do so. The abovementioned Huyghens is entirely occupied
in polishing lenses. He has fitted up for the purpose a handsome workshop,
in which he can also construct moulds. What will be the result I know
not, nor, to speak the truth, do I greatly care. Experience has sufficiently
taught me, that the free hand is better and more sure than any machine
for polishing spherical moulds. I can tell you nothing certain as yet
about the success of the clocks or the date of Huyghens' journey to France.
Baruch Spinoza: Correspondence / 32 (15)