In this recognition of objective and universally valid relations existing among ideas, which the thinking subject, through comparisons voluntarily instituted, discovers valid or finds given, but which it can neither alter nor demur to, Locke abandons empirical ground (cf. p. 155) and approaches the idealists of the Platonizing type.  His inquiry divides into two very dissimilar parts (a psychological description of the origin of ideas and a logical determination of the possibility and the extent of knowledge), the latter of which is, in Locke’s opinion, compatible with the former, but which could never have been developed from it.  The rationalistic edifice contradicts the sensationalistic foundation.  Locke had hoped to show the value and the limits of knowledge by an inquiry into the origin of ideas, but his estimate of this value and these limits cannot be proved from the a posteriori origin of ideas—­it can only be maintained in despite of this, and stands in need of support from some (rationalistic) principle elsewhere obtained.  Thinkers who trace back all simple ideas to outer and inner perception we expect to reject every attempt to extend knowledge beyond the sphere of experience, to declare the combinations of ideas which have their origin in sensation trustworthy, and those which are formed without regard to perception, illusory; or else, with Protagoras, to limit knowledge to the individual perceiving subject, with a consequent complete denial of its general validity.  But exactly the opposite of all these is found in Locke.  The remarkable spectacle is presented of a philosopher who admits no other sources of ideas than perception and the voluntary combination of perceptions, transcending the limits of experience with proofs of the divine existence, viewing with suspicion the ideas of substance formed at the instance of experience, and reducing natural science to the sphere of mere opinion; while, on the other hand, he ascribes reality and eternal validity to the combinations of ideas formed independently of perception, which are employed by mathematics and ethics, and completely abandons the individualistic position in his naive faith in the impregnable validity of the relations of ideas, which is evident to all who turn their attention to them.  The ground for the universal validity of the relations among ideas as well as of our knowledge of them, naturally lies not in their empirical origin (for my experience gives information to me alone, and that only concerning the particular case in question), but in the uniformity of man’s rational constitution.  If two men really have the same ideas—­not merely think they have because they use similar language—­it is impossible, according to Locke, that they should hold different opinions concerning the relation of their ideas.  With this conviction, that the universal validity of knowledge is rooted in the uniformity of man’s rational constitution, and the further one, that we attain certain knowledge only when things