34 J. S. HALDANE. aware. We have already seen why a living organism and its surroundings must be regarded as a system of parts reciprocally determining one another. But when the sun is treated as a source of energy and as such considered in its relation to life, it is really taken for granted at the outset that life is a mechanical process. For if one phenomenon is brought under a certain category, all other phenomena are potentially brought under the same category. It is not possible without contradiction to regard phenomena under two categories simultaneously. 1 Therefore the form of the above objection precludes the possibility of an answer to it. All that can be said with regard to it is that in relation to the essential characteristics of life, the conception of the sun as a source of energy may be shown to be self-contradictory, in the same way as was done with the conception of the organism as a machine. The range of application of a category cannot be limited in space ; so that in bringing the phenomena of life under the category of reciprocity we do not limit the application of this category to a certain por- tion of space, leaving an outside region in which the category of cause and effect holds undisputed sway. Hence we may regard the sun as participating, inasmuch as it is a part of the surroundings, in the system of life of an organism, but not as acting on that system from without. It follows from this that the surroundings of an organism are to be looked on, not merely as objects in ambient space which may act on, or else be acted on by, the organism, but as these objects in so far as they participate with other objects in forming with the organism a system whose parts reciprocally determine one another. The mode of life of organisms on the earth, for instance, is suited to the present condition of the solar system, and so indirectly to conditions beyond. But it is not enough simply to say that these conditions modify the life of terrestrial organisms, since that life actively adapts itself to them. The relation is that of reciprocal action : not merely that of cause and eifect. So long as it is allowed to appear that, apart from a consideration of life, the category of cause and effect has an absolute validity of its own, the foregoing reasoning will seem incomplete in some sense. For the world as it is for physical science the world of causes and effects seems to have no definite relation to the same world regarded, in relation to life, under the category of reciprocity. The task undertaken in this article is not, however, to inquire how 1 This point is worked out in Essays in Philos^hical Criticism, ii.
