As, therefore, in my opinion the immense importance of Darwin’s theory lies in the fact that it has _mechanically explained those organic phenomena of forms_ which had hitherto been unexplained, it is perhaps necessary that I should here say a few words about the different ideas connected with the word “explanation.” It is very frequently said, in opposition to Darwin’s theory, that it does indeed explain those phenomena by Inheritance and Adaptation, but that it does not at the same time explain those properties of organic matter, and that therefore we do not arrive at first causes. This objection is quite correct, but it applies equally to _all_ explanations of phenomena. We _nowhere_ arrive at a knowledge of first causes. The origin of every simple salt crystal, which we obtain by evaporating its mother liquor, is no less mysterious to us, as far as concerns its first cause, and in itself no less incomprehensible than the origin of every animal which is developed out of a simple cell. In explaining the most simple physical or chemical phenomena, as the falling of a stone, or the formation of a chemical combination, we arrive, by discovering and establishing the active causes—for example, the gravitation or the chemical affinity—at other remoter phenomena, which in themselves are mysterious. This arises from the limitation or relativity of our powers of understanding. We must not forget that human knowledge is absolutely limited, and possesses only a relative extension. It is, in its essence, limited by the very nature of our senses and of our brains. All knowledge springs from sensuous perceptions. In opposition to this statement, the innate, _à priori_ knowledge of man may be brought up; but we can see that the so-called _à priori_ knowledge can by Darwin’s theory be proved to have been acquired _à posteriori_, being based on experience as its first cause. Knowledge which is based originally upon purely empirical observations, and which is therefore a purely sensuous experience, but has then been transmitted from generation to generation by inheritance, appears in later generations as if it were independent, innate, and _à priori_. In our late animal ancestors, all our so-called “_à priori_ knowledge” was originally acquired _à posteriori_, and only gradually became _à priori_ by inheritance. It is based in the first instance upon experiences, and by the laws of Inheritance and Adaptation we can positively prove that knowledge _à priori_ and knowledge _à posteriori_ cannot rightly be placed in opposition, as is usually done. On the contrary, sensuous experience is the original source of _all_ knowledge. For this reason alone, all our knowledge is limited, and we can never apprehend the _first causes_ of any phenomena. The force of crystallization, the force of gravitation, and chemical affinity remain in themselves just as incomprehensible as do Adaptation and Inheritance.