A purposive adaptation to environment with consequent deviation from uniformity in reaction would necessarily escape our notice if it took place with the rapidity of the beat of a gnat’s wing, or again, if it required centuries for its establishment.[135] Other similar subjective conditions which would necessarily cut us off from the recognition of purposive fresh adaptations widely different from those which occur in our own life, are the limitations of our power of attending to more than a certain number of presentations simultaneously; and again, the restriction of our sense-perception to a few types, and the impossibility of perceiving contents belonging to those types when they fall below or above the lower and upper “thresholds” of sensibility. These considerations do not, of course, positively prove that the routine uniformity of natural processes is only subjective appearance, but they are sufficient to show that there is no valid reason for taking it to be more, and in conjunction with our previous positive argument for the sentient individuality of all real existence, they suffice to bring our general interpretation of the physical order under Mr. Bradley’s canon that “What must be and can be, that _is_.” § 4. (3) What, then, are we to make of the principle of the “Uniformity of Nature”? Any principle which does actual work in science must somehow be capable of justification, and if our interpretation of the physical order really conflicts with a fundamental scientific principle, it must contain fallacy somewhere. Fortunately, there is no real conflict. In dealing with the principle of Uniformity, we must distinguish very carefully between the sense in which it is actually required for the purposes of science and the sense which has been put upon it in the set of metaphysical doctrines popularly but illogically deduced from the actual procedure of the sciences. As we have seen already, it is impossible to affirm the principle of Uniformity as an axiom of systematic thought. It is also not capable of verification as an empirical truth. Its logical character must therefore be that of a postulate, an assumption defensible on the ground of practical usefulness, but only so far as it actually succeeds.