We find this transcendental presupposition lurking in different forms in the principles of philosophers, although they have neither recognized it nor confessed to themselves its presence.That the diversities of individual things do not exclude identity of species, that the various species must be considered as merely different determinations of a few genera, and these again as divisions of still higher races, and so on- that, accordingly, a certain systematic unity of all possible empirical conceptions, in so far as they can be deduced from higher and more general conceptions, must be sought for, is a scholastic maxim or logical principle, without which reason could not be employed by us.For we can infer the particular from the general, only in so far as general properties of things constitute the foundation upon which the particular rest.
That the same unity exists in nature is presupposed by philosophers in the well-known scholastic maxim, which forbids us unnecessarily to augment the number of entities or principles (entia praeter necessitatem non esse multiplicanda).This maxim asserts that nature herself assists in the establishment of this unity of reason, and that the seemingly infinite diversity of phenomena should not deter us from the expectation of discovering beneath this diversity a unity of fundamental properties, of which the aforesaid variety is but a more or less determined form.This unity, although a mere idea, thinkers have found it necessary rather to moderate the desire than to encourage it.It was considered a great step when chemists were able to reduce all salts to two main genera- acids and alkalis; and they regard this difference as itself a mere variety, or different manifestation of one and the same fundamental material.
The different kinds of earths (stones and even metals) chemists have endeavoured to reduce to three, and afterwards to two; but still, not content with this advance, they cannot but think that behind these diversities there lurks but one genus- nay, that even salts and earths have a common principle.It might be conjectured that this is merely an economical plan of reason, for the purpose of sparing itself trouble, and an attempt of a purely hypothetical character, which, when successful, gives an appearance of probability to the principle of explanation employed by the reason.But a selfish purpose of this kind is easily to be distinguished from the idea, according to which every one presupposes that this unity is in accordance with the laws of nature, and that reason does not in this case request, but requires, although we are quite unable to determine the proper limits of this unity.
If the diversity existing in phenomena- a diversity not of form (for in this they may be similar) but of content- were so great that the subtlest human reason could never by comparison discover in them the least similarity (which is not impossible), in this case the logical law of genera would be without foundation, the conception of a genus, nay, all general conceptions would be impossible, and the faculty of the understanding, the exercise of which is restricted to the world of conceptions, could not exist.The logical principle of genera, accordingly, if it is to be applied to nature (by which I mean objects presented to our senses), presupposes a transcendental principle.In accordance with this principle, homogeneity is necessarily presupposed in the variety of phenomena (although we are unable to determine a priori the degree of this homogeneity), because without it no empirical conceptions, and consequently no experience, would be possible.
The logical principle of genera, which demands identity in phenomena, is balanced by another principle- that of species, which requires variety and diversity in things, notwithstanding their accordance in the same genus, and directs the understanding to attend to the one no less than to the other.This principle (of the faculty of distinction) acts as a check upon the reason and reason exhibits in this respect a double and conflicting interest- on the one hand, the interest in the extent (the interest of generality) in relation to genera; on the other, that of the content (the interest of individuality) in relation to the variety of species.In the former case, the understanding cogitates more under its conceptions, in the latter it cogitates more in them.This distinction manifests itself likewise in the habits of thought peculiar to natural philosophers, some of whom- the remarkably speculative heads- may be said to be hostile to heterogeneity in phenomena, and have their eyes always fixed on the unity of genera, while others- with a strong empirical tendency- aim unceasingly at the analysis of phenomena, and almost destroy in us the hope of ever being able to estimate the character of these according to general principles.
The latter mode of thought is evidently based upon a logical principle, the aim of which is the systematic completeness of all cognitions.This principle authorizes me, beginning at the genus, to descend to the various and diverse contained under it; and in this way extension, as in the former case unity, is assured to the system.