necessary relation to each other, rests only in it, is in fact merely relative; but of this more presently. I have further shown, that the necessary relation which the principle of sufficient reason expresses generally, appears in other forms corresponding to the classes into which objects are divided, according to their possibility; and again that by these forms the proper division of the classes is tested. I take it for granted that what I said in this earlier essay is known and present to the reader, for if it had not been already said it would necessarily find its place here. § 3. The chief distinction among our ideas is that between ideas of perception and abstract ideas. The latter form just one class of ideas, namely concepts, and these are the possession of man alone of all creatures upon earth. The capacity for these, which distinguishes him from all the lower animals, has always been called reason.(5) We shall consider these abstract ideas by themselves later, but, in the first place, we shall speak exclusively of the _ideas of perception_. These comprehend the whole visible world, or the sum total of experience, with the conditions of its possibility. We have already observed that it is a highly important discovery of Kant’s, that these very conditions, these forms of the visible world, _i.e._, the absolutely universal element in its perception, the common property of all its phenomena, space and time, even when taken by themselves and apart from their content, can, not only be thought in the abstract, but also be directly perceived; and that this perception or intuition is not some kind of phantasm arising from constant recurrence in experience, but is so entirely independent of experience that we must rather regard the latter as dependent on it, inasmuch as the qualities of space and time, as they are known in _a priori_ perception or intuition, are valid for all possible experience, as rules to which it must invariably conform. Accordingly, in my essay on the principle of sufficient reason, I have treated space and time, because they are perceived as pure and empty of content, as a special and independent class of ideas. This quality of the universal forms of intuition, which was discovered by Kant, that they may be perceived in themselves and apart from experience, and that they may be known as exhibiting those laws on which is founded the infallible science of mathematics, is certainly very important. Not less worthy of remark, however, is this other quality of time and space, that the principle of sufficient reason, which conditions experience as the law of causation and of motive, and thought as the law of the basis of judgment, appears here in quite a special form, to which I have given the name of the ground of being. In time, this is the succession of its moments, and in space the position of its parts, which reciprocally determine each other _ad infinitum_.