The faculty of thought produces in contact with sense perceptions that which manifests itself as true nature, as a general truth. A primitive concept accomplishes this at first only instinctively, while a scientific concept is a conscious and voluntary repetition of this primitive act. Scientific understanding wanting to know an object, such as for instance heat, is not hunting after the phenomena themselves. It does not aim to see or hear how heat melts iron or wax, how it benefits in one case or injures in another, how it makes eggs solid or ice liquid, nor does it concern itself with the difference between the heat of an animal, of the sun, or of a stove. All these things are from the point of view of the faculty of understanding, only effects, phenomena, qualities. It desires to get at the essence, the true nature of things, it strives to find a general law, a concise scientific extract, of things seen, heard, and felt. The abstract nature of things cannot be a tangible object. It is a concept of theory, of science, of the faculty of thought. The understanding of heat consists in singling out that which is common to all phenomena of heat, which is essential or true for all heat. _Practically the nature of heat consists of the sum total of all its manifestations, theoretically in its concept, scientifically in the analysis of this concept. To analyze the concept of heat means to ascertain that which is common to all manifestations of heat._ The general nature of the thing is its true nature, the general quality its true quality. We define rain more truly as being wet than as being fertilizing, because it gives moisture wherever it falls, while it fertilizes only under certain circumstances and in certain places. My true friend is one who is constant and loyal to me all my life under all circumstances. Of course, we must not believe in any absolute and unconditional friendship any more than in any absolute and eternal truth. Perfectly true, perfectly universal, is only the general existence, the universe, the absolute quantity. But the real world is absolutely relative, absolutely perishable, an infinity of manifestations, an infinity of qualities. All truths are simply parts of this world, partial truths. Semblance and truth flow dialectically into one another like hard and soft, good and bad, right and wrong, but at the same time they remain different. Even though I know that there is no rain which is "fertile in itself," and no friend who is true in an absolute sense, I may nevertheless refer to a certain rain as fertile in relation to certain crops, and I may distinguish between my more or less true friends.