But if the particular effect, of which we enquire the reason, does not appear to depend on these general effects, nor to have any analogy with other known effects, then this effect, being the only one of its kind, and having nothing in common with other effects, at least known to us, the question is insolvable: because, not having, in this point, any known subject which has any connection with that we would explain, there is nothing from whence we can draw the reason sought after. When the reason of a general cause is demanded, it is unanswerable, because it exists in every object; and, on the other hand, the reason of a singular or isolated effect is not found, because not any thing known has the same qualities. We cannot explain the reason of a general effect, without discovering one more general; whereas the reason of an isolated effect may be explained by the discovery of some other relative effect, which although we are ignorant of at present, chance or experience may bring to light.
Besides these, there is another kind of question, which may be called, the question of fact. For example. Why do trees, dogs, &c. exist? All these fact questions are totally insoluble, for those who answer them by final causes do not consider that they take the effect for the cause; the connection particular objects have with us having no influence on their origin. Moral affinity can never become a physical reason.
We must carefully distinguish these questions where the why is used, from those where the how is employed, and more so from those where the how many is mentioned. Why is always relative to the cause of the effect, or to the effect itself. How is relative to the mode from which the effect springs, and the how many has relation only to the proportionate quantity of the effect.
All these distinctions being explained, let us proceed to examine the question concerning the reproduction of bodies. If it is asked, why animals and vegetables reproduce? we shall clearly discover, that this being a question of fact, it is insolvable, and useless to endeavour at the solution of it. But if it is asked, how animals and vegetables reproduce; we reply by relating the history of the generation of every species of animal, and of the reproduction of each distinct vegetable; but, after having run over all the methods of an animal engendering its resemblance, accompanied even with the most exact observations, we shall find it has only taught us facts without indicating causes; and that the apparent methods which Nature makes use of for reproduction, do not appear to have any connection with the effects resulting therefrom; we shall be still obliged to ask, what is the secret mode by which she enables different bodies to propagate their own species?
This question is very different from the first and second; it gives liberty of enquiry and admits the employment of imagination, and therefore is not insolvable, for it does not immediately belong to a general cause; nor is it entirely a question of fact, for provided we can conceive a mode of reproduction dependent upon, or not repugnant to, original causes, we shall have gained a satisfactory answer; and the more it shall have a connection with other effects of nature, the better foundation will it be raised upon.
By the question itself it is, therefore, permitted to form hypotheses, and to select that which shall appear to have the greatest analogy with the other phenomena of nature. But we must exclude from the number all those which supposes the thing already done; for example, such as suppose that all the germs of the same species were contained in the first seed, or that every reproduction is a new creation, and immediate effect of the Almighty's will; because these hypotheses are questions of fact, and on which it is impossible to reason. We must also reject every hypothesis which might have final causes for its object; such as, we might say, that reproduction is made in order for the living to supply the place of the dead, that the earth may be always covered with vegetables, and peopled with animals; that man may find plenty for his subsistence, &c. because these hypotheses, instead of explaining the effects by physical causes, are founded only on arbitrary connections and moral agreements. At the same time we must not rely on these absolute axioms and physical problems, which so many people have improperly made use of, as principles; for example, there is no fecundation made apart from the body, nulla fœcondatio extra corpus; every living thing is produced from an egg; all generation supposes sexes, &c. We must not take these maxims in an absolute sense, but consider them only as signifying things generally performed in one particular mode rather than in any other.
Let us, therefore, search after an hypothesis which has not any of those defects, and by which we cannot fall into any of these inconveniences; if, then, we do not succeed in the explanation of the mechanical power Nature makes use of to effect the reproduction of beings, we shall, at least, arrive at something more probable than what has hitherto been advanced.
As we can make moulds, by which we can give to the external parts of bodies whatever figure we please, let us suppose Nature can form the same, by which she not only bestows on bodies the external figure but also the internal. Would not this be one mode by which reproduction may be performed?
Let us, then, consider on what foundation this supposition is raised: let us examine if it contains any thing contradictory, and afterwards we shall discover what consequences may be derived from it. Though our senses are only judges of the external parts of bodies, we perfectly comprehend external affection and different figures. We can also imitate Nature, by representing external figures by different modes, as by painting, sculpture, and moulds; but although our senses are only judges of external qualities, we know there are internal qualities, some of which are general, as gravity. This quality, or power, does not act relatively to surfaces, but proportionably to the masses, or quantities of matter; there is, therefore, very active qualities in Nature, which even penetrate bodies to the most internal parts; but we shall never gain a perfect idea of these qualities, because, not being external, they cannot fall within the compass of our senses; but we can compare their effects, and deduce analogies therefrom, to answer for the effect of similar qualities.