Science and philosophy have for centuries been sustained by unquestioning faith in perception. Perception opens a window on to things. This means that it is directed, quasi-teleologically, towards a *truth in itself* in which the reason underlying all appearances is to be found. The tacit thesis of perception is that at every instant experience can be co-ordinated with that of the previous instant and that of the following, and my perspective with that of other consciousnesses--that all contradictions can be removed, that monadic and intersubjective experience is one unbroken text--that what is now indeterminate for me could become determinate for a more complete knowledge, which is as it were realized in advance in the thing, or rather which is the thing itself. Science has first been merely the sequel or amplification of the process which constitutes perceived things. Just as the thing is the invariant of all sensory fields and of all individual perceptual fields, so the scientific concept is the means of fixing and objectifying phenomena. Science defined a theoretical state of bodies not subject to the action of any force, and *ipso facto* defined force, reconstituting with the aid of these ideal components the processes actually observed. It established statistically the chemical properties of pure bodies, deducing from these those of empirical bodies, and seeming thus to hold the plan of creation or in any case to have found a reason immanent in the world. The notion of geometrical space, indifferent to its contents, that of pure movement which does not by itself affect the properties of the object, provided phenomena with a setting of inert existence in which each event could be related to physical conditions responsible for the changes occurring, and therefore contributed to this freezing of being which appeared to be the task of physics. In thus developing the concept of the thing, scientific knowledge was not aware that it was working on a presupposition. Precisely because perception, in its vital implications and prior to any theoretical thought, is presented as perception of a being, it was not considered necessary for reflection to undertake a genealogy of being, and it was therefore confined to seeking the conditions which make being possible. Even if one took account of the transformations of determinant consciousness, even if it were conceded that the constitution of the object is never completed, there was nothing to add to what science said of it; the natural object remained an ideal unity for us and, in the famous words of Lachelier, a network of general properties. It was no use denying any ontological value to the principles of science and leaving them with only a methodical value, for this reservation made no essential change as far as philosophy was concerned, since the sole conceivable being remained defined by scientific method. The living body, under these circumstances, could not escape the determinations which alone made the object into an object and without which it would have had no place in the system of experience. The value predicates which the reflecting judgment confers upon it had to be sustained, in being, by a foundation of physico-chemical properties. In ordinary experience we find a fittingness and a meaningful relationship between the gesture, the smile and the tone of a speaker. But this reciprocal relationship of expression which presents the human body as the outward manifestation of a certain manner of being-in-the-world, had, for mechanistic physiology, to be resolved into a series of causal relations.