Is it difficult to discover that our ideas proceed only from our senses? that the things we look on as real and existing are those of which our senses have always rendered us the same testimony? that those which we conceive to have certain existence are those which ever present themselves in the same order? that consequently our ideas, very far from being the causes of things, are only effects, and so far from resembling particular things, become less similar to the objects as they are more general; that at length our mental abstractions are only negative beings, which do not exist even intellectually but by the retrenchment which we make of sensible qualities to real beings.
From hence is it not plain that abstractions can never become principles, neither of existence nor real knowledge? on the contrary, our knowledge can only proceed from the results of properly comparing our sensations. These results are what is termed experience, the sole source of all real science. The adoption of every other principle is an abuse, and every edifice built on abstracted ideas is a temple founded on error.
Error bears a much more extended signification in philosophy than in morality: in morals a thing may be false, only because it is misrepresented. Metaphysical falsehood consists not in misrepresentation alone, but in crediting that which has no existence, and even in not being of any mode whatever. It is in this kind of error, of the first order, that the Platonists, the Sceptics, and the Egotists have fallen into, their false suppositions have obscured the natural light of truth, clouded reason, and retarded the advancement of philosophy.
The second principle made use of by Plato, and by most of the speculative philosophers, is a final cause. Nevertheless, to reduce this principle to its just value, a single moment of reflection is only requisite. To say there is light because we have eyes, and sounds because we have ears, or to say that we have ears and eyes because there is light and sound, is it not exactly the same thing? shall we ever discover any thing by this mode of explanation? Is it not evident that final causes are only arbitrary relations and moral abstractions, which should impose on us still less than metaphysical abstractions, because their origin is less noble and a more false supposition; and although Leibnitz has endeavoured to raise this principle to the highest degree by the name of sufficient reason, and Plato has represented it by the most flattering portrait, under the title of perfection, yet it cannot prevent our seeing it as trifling and precarious. Are we better acquainted with the effects of Nature, from being told that nothing is made without a reason, or that all is made in view of perfection? What is this sufficient reason? what is this perfection? are they not moral beings created by intellects purely human? are they not arbitrary relations which we have generalized? on what are they founded? on moral affinities which, far from producing any physical or real existence, only alter the reality and confound the objects of our sensations, perceptions and knowledge, with those of our sentiments, our passions and our wills.
I could adduce many arguments on this subject, but I do not pretend to make a treatise on philosophy, and shall return to physics, from which the ideas of Plato on universal generation made me digress. Aristotle, who was as great a philosopher as Plato and a much better physician, instead of losing himself in the region of hypotheses, relied, on the contrary, on collected facts, and speaks in a more intelligible language.
Matter, which is only a capacity of receiving forms, takes in generation a form like that of the individual which furnishes it; and with respect to the generation of animals that have sexes, he thinks that the male alone furnishes the prolific principle, and that the female affords nothing that can be looked upon as such.[BM] For though he says elsewhere, speaking of animals in general, that the female emits a seminal fluid within herself, yet he does not regard that as a prolific principle: nevertheless, according to him, the menstrual blood serves for the formation, growth, and nutriment of the fœtus, but the efficient principles exist only in the seminal fluid of the male, which does not act like matter, but as the cause. Averrhois, Avicenna, and other philosophers, who followed the sentiments of Aristotle, have sought for reasons to prove that females have no prolific fluid; they urge, that as females have a menstrual fluid that was necessary and sufficient for generation, it does not appear natural to suppose they possess any other; particularly because it begins to appear, like the seminal fluid in the males, at the age of puberty; besides, continue they, if females have really a seminal and prolific fluid, why do they not produce without the approach of the male, since they contain the prolific principle as well as the matter necessary for the nutriment and growth of the embryo? This last reason seems to be the only one which merits any attention. The menstrual blood seems to be necessary for the support, nutriment, and growth of the fœtus, but it can have no part in the first formation, which is made by the mixture of two fluids alike prolific. Females therefore may have, as well as the males, a prolific fluid for the formation of the embryo, besides the menstrual blood for its nutriment and expansion; and certainly a female being possessed of a prolific fluid, extracted from all parts of her body, as well as the necessary means of nourishment and expansion, it is no impossible imagination that she would produce females without any communication with the male. It must be allowed, that this metaphysical reasoning which the Aristotelians adopt to prove that females have no prolific fluid, may become the most considerable objection that can be made against all systems of generation, and particularly against our explanation.
[BM] See Aristotle, de gen. lib. i. cap. 20 and lib. xi. cap. 4.
Let us suppose, it may be said, as you have attempted to prove, that the superfluous organic molecules are sent back into the testicles and seminal vessels of the male, why, by the power of your supposed attracting forces, do they not form small organized beings, perfectly resembling the male? and for the same reason similar beings in the female? If you answer, that there is an appearance that the liquor of the male contains only males, and that of the female only females, but that all these perish for want of the necessary means for expansion, and that there are only those formed by the mixture of both which can expand and come into the world; may we not be asked why this mode of generation, which is the most complicated, difficult, and least abundant, is that which Nature prefers in so striking a manner, that almost all animals multiply by this mode of communication of the male with the female?
I shall content myself at present with answering, that the fact is such as we have represented it; the objection becomes a fact question; to which, as we have observed, there is no other solution to be given than that of the fact itself. It may be insisted, it is the most complicated mode of production; yet this mode, which appears the most complicated to us, is certainly the most simple for nature, because, as we have remarked, what happens the most often, however difficult it may appear to our ideas, must in reality be the most simple; which does not prevent us from conceiving it to be complex, as we judge of it according to that knowledge which our senses and reflections can give us thereon.
The assertion of the Aristotelians, that females have no prolific fluid, must fall to the ground, if we pay attention to the resemblance of children to their mothers, of mules to the female that produces them, of mongrels and mulattos, all of which resemble more the mother than the father. If, besides these, we consider the organs of females are, like those of the males, formed so as to prepare and receive the seminal fluid, we shall be readily persuaded that such a fluid must exist, whether it resides in the spermatic vessels, the testicles, or in the matrix: or whether it issues, when provoked, by the passages of de Graaf, situated at the neck, and near the external orifice of the urethra.