To the riddles which nature propounds to us," said Lord Salisbury in his Oxford address to the British Association, " the confession of ignorance must constantly be our only reasonable answer." And the saying was hailed with acclamation; not least among those who most strenuously repudiated agnosticism. This cheerful acceptance of ignorance, however, is not the attitude of the inquiring spirit. It is true that we should always be ready to say: Here is a gap in our assured knowledge; we can only complete our ideal scheme by filling in the gap with hypothesis or, it may be, conjecture. But the confession of ignorance, though it may be an honest admission of failure, is scarcely a reasonable answer to a riddle. I have no desire to quibble over phrases and modes of expression. Lord Salisbury's meaning is clear enough. And the modesty that he desired to advocate is an altogether admirable quality. But the attitude of science towards ignorance is always one of intolerance and profound dissatisfaction, save in so far as it affords opportunities for discovery. And even the agnostic has, I conceive, to school himself with sorrowful diligence into the acceptance of the fact that the riddle of the universe can never be answered by beings of such limited capacities as are found in man.

Whether we regard ignorance, however, with complacency or intolerance, whether a confession of failure in presence of unsolved problems is a reasonable answer to the riddles or a modest confession that so far we have guessed in vain, the fact remains that our interpretations of nature are still, and are likely long to remain, in many respects faulty and inadequate, lacking in precision and lacking in comprehensiveness, mere outline sketches at best rather than finished pictures. Even the last half-century's splendid advances in science of which we are so proud, and justly proud, have served not only to reveal how little we knew fifty years ago, but how great is the array of new problems which still await solution. It seems literally true that the more we achieve in the mastery of knowledge, the more fully do we realise the range and extent of the as yet unachieved. Must we then wait indefinitely for further and fuller achievement before we attempt to express in synthetic form our interpretations of nature? Surely this is neither possible for the inquiring mind, constituted as it is, nor desirable. Rather should we build, as we may and can, from time to time, with the available materials for construction, remembering that when we have done our best to-day there must be many imperfections which the broader outlook and deeper insight of the future will most assuredly disclose.

At the outset of this essay I drew attention to the fact that divergent interpretations arise according as the course pursued is from within outwards or from without inwards. The purposes of human life are for primitive folk the centres of predominant attention. Continually thrust into the foreground of daily experience, they are naively accepted as the basis of all explanation. Taken for granted as given factors in social life, no need is felt to explain that, in terms of which all explanations are afforded. Nothing independent of the practical purposes of man has any interest in and for itself. If incidentally given in experience, it is regarded as a negligible factor. But gradually there arose the perception that, not only were the events which happened in the world around closely connected with human well-being, but they were also intimately connected among themselves; and a knowledge of their external inter-relationships was found to be of value in furthering and rendering more efficient the application of human endeavour. Herein, however, lay the germs of a decentralising process. Interpretations, proceeding hitherto wholly on the method of projecting human purpose on to the plane of nature, now proceeded partly on the method of introjecting the mechanical explanations of the outer world into the life and eventually into the very soul of man. The culmination of the latter process is modern naturalism, according to which human purpose is itself completely explained introjectively as the product of an evolution whose dynamic germs were contained in a primitive fire-mist or a swarm of meteoric particles. The question therefore arises: Does the later interpretation of nature in terms of mechanism, even if we grant its premises and its conclusions, entirely supersede and render invalid the earlier interpretation in terms of purpose?

The naturalistic basis of experience then claimed our attention. This basis is afforded by our impressions and perceptions. I urged that on this plane of thought—within this universe of discourse—the test of reality is correspondence of objective reference. If this holds good not only for the individual during different modes of perception, but also for mankind, it has complete validity for all practical purposes. I claimed for this objective reality and validity, that it holds good for experience down to its minutest details. The fire is in this sense really hot, snow is really cold, the apple is really scented and acid-sweet, the varied colours of sunset, shading through many and varied tints, are actually parts of objective existence, no less than the shape and solidity of this desk at which I write. But a percipient, actually or ideally present, is throughout assumed on this interpretation when it comes to philosophical maturity. Always and only for perception, for experience, does the validity claimed for the world of objects hold good. Before it reaches this final expression, however, the naturalistic interpretation has to pass through several stages of development. At first it is naively assumed that, since the object of experience, just as it stands, is independent of this, that, and the other percipient, taken severally, it is absolutely independent of all experience. It is real not only for us but in and for itself. For practical purposes, this additional hypothesis, though wholly unnecessary, is quite convenient; and there can obviously be nothing in the data of experience to contradict it. But when it is submitted to critical analysis, difficulties in the way of its thorough-going acceptance inevitably arise. Sweetness and colour, for example, are clearly, it is urged, dependent on our organs of sense. These are not properties existent as such in the external object, but rather modes in which certain real properties affect us, organised as we are. Thus arose the distinction between the primary qualities existent independently of any percipient, and the secondary qualities dependent on the way in which the external mechanism affects certain kinds of living tissue in the organs of special sensation. But how come we to have any knowledge of this mechanism—this really existent matter in motion? Only through perception; only through other channels of sense. Critical analysis, therefore, pushing home its cross-examination of experience, shows that all modes of that experience stand on precisely the same footing. Primary and secondary qualities alike are in the same category as dependent on experience. All are equally valid and real on the plane of our practical dealings with the world, though some must be translated into physical terms for exact mathematical treatment. Of any independent existence, however, experience, so long as it is restricted to the data afforded by our perceptions, can give no information. Even if we postulate such an existence, as the source and origin of our experience, it cannot be self-existent under the same guise as that under which it is manifested in phenomena. But that which gives to experience its practical guidance and its intellectual value is purpose. This cannot be directly presented to experience, though it may perchance be felt within it, or postulated as the ground of its being. May it not therefore be a rational, though not an empirical, conclusion that the underlying source of objective existence is the purpose of which it is the expression?