As the categories of substance and causality are specially preferred to the others by Kant and the Kantians, and are even proclaimed by some as the only fundamental concepts, so also the principles of relation have an established reputation for special importance.  The leading ideas in the proofs of the “Analogies of Experience”—­for in spite of their underivative character the principles require, and are capable of, proof—­may next be noted.

The time determinations of phenomena, the knowledge of their duration, their succession, and their coexistence, form an indispensable part of our experience, not only of scientific experience, but of everyday experience as well.  How is the objective time-determination of things and events possible?  If the matter in hand is the determination of the particulars of a fight with a bloody ending, the witnesses are questioned and testify:  We heard and saw how A began the quarrel by insulting B, and the latter answered the insult with a blow, whereupon A drew his knife and wounded his opponent.  Here the succession of perceptions on the part of the persons present is accepted as a true reproduction of the succession of the actual events.  But the succession of perceptions is not always the sure indication of an actual succession:  the trees along an avenue are perceived one after the other, while they are in reality coexistent.  We might now propose the following statement:  The representation of the manifold of phenomena is always successive, I apprehend one part after another.  I can decide whether these parts succeed one another in the object also, or whether they are coexistent, by the fact that, in the second case, the series of my perceptions is reversible, while in the first it is not.  I can, if I choose, direct my glance along the avenue in such a way that I shall begin the second time with the tree at which I left off the first time; if I wish to assure myself that the parts of a house are coexistent, I cause my eye to wander from the upper to the lower portions, from the right side to the left, and then to perform the same motions in the opposite direction.  On the other hand, it is not left to my choice to hear the thunder either before or after I see the lightning, or to see a passing wagon now here, now there, but in these cases I am bound in the succession of my sensuous representations.  The possibility of interchange in the series of perceptions proves an objective coexistence, the impossibility of this, an objective succession.  But this criterion is limited to the immediate present, and fails us when a time relation between unobserved phenomena is to be established.  If I go at evening into the dining room and see a vessel of bubbling water, which is to be used in making tea, over a burning spirit lamp, whence do I derive the knowledge that the water began, and could begin, to boil only after the alcohol had been lighted, and not before?  Because I have often seen the flame precede the boiling of the water,