Let us determine well the meaning of the words, and this difficulty will vanish. An immaterial being is not the same as a spirit; every spirit is immaterial, but not every thing immaterial is a spirit. _Immaterial_ denotes negation of matter; spirit implies more than this; for we understand by it a simple being endowed with understanding and free-will. The soul of brutes is then immaterial, but not a spirit. Some one may say, that what is not body is spirit, that between these two classes of beings there is no medium. But why? Whence such certainty? If it be said that there is no medium between the material and the immaterial, this is true; for there is, in truth, no medium between yes and no; every thing either is or is not; but there enters into the idea of spirit much more than the simple negation of matter; there is the idea of an active, intelligent, and free principle. 16. It may then be asked: wherein consists the nature of the soul of brutes? And we ask, wherein consists the nature of the greater part of the things which we perceive? Do we know this nature in itself or in its acts? Do we see our own soul intuitively? Or do we, perchance, know it by the acts of which we are conscious? If so, we know in like manner the sensitive soul by its acts, that is, by perception of the senses; we know that it is not matter, for matter is incapable of sensation; and the reasons which show us that our soul is a simple being, an active principle endowed with understanding and free-will enable us to say that the soul of brutes is a simple being endowed with the faculty of feeling, and with instincts and appetites of the sensible order. We know not what this active principle in itself considered is, but its acts show it to be a force superior to bodies, one of the many activities which are the life of nature. We encounter this living force in a portion of matter admirably organized; the end of this organization is the harmonious exercise of the faculties of that living being which we call animal. Not to know what this force in itself is, does not prevent us from affirming its existence, for phenomena reveal it to us in an indisputable manner. 17. What, then, will be the fate of these souls, or living forces, if the organization which gives them life be destroyed? Will they be reduced to non-existence, since, not being composed of parts, they cannot be decomposed? Will they continue to exist until their turn shall come to preside over a new organization? It will be well to separate these various questions, and examine them apart.