Scientists continue their researches as before; the universities flourish; discovery follows discovery. And the theorists of knowledge themselves do not spend sleepless nights in the endeavour to find new justifications for science. Yet, I repeat, though they can come to an understanding about practically nothing else, they amaze us by their unanimity upon this point--they are all convinced that it is their duty to justify science and exalt her. So that the modern theory of knowledge is no longer a science, but an apology. And its demonstrations are like those of apology. Once science must be defended, it is necessary to begin by praising her, that is by selecting evidence and data to show that science fulfils some mission or other, but indubitably a very high and important one, or, on the other hand, by painting a picture of the fate that would overtake mankind, if it was deprived of science. Thus the apologetic element has begun to play almost as large a part in the theory of knowledge as it has done hitherto in theology. Perhaps the time is at hand when scientific apologetics will be officially recognised as a philosophic discipline. But, _qui s'excuse s'accuse._ It is plain that all is not well with science, since she has begun to justify herself. Besides, apologetics are only apologetics, and sooner or later the theory of knowledge will be tired of psalms of praise, and will demand a more complex and responsible task, and a real labour. At present the theorists start with the assumption that scientific knowledge is perfect knowledge, and therefore the premisses upon which it is builded are not subject to criticism. The law of causation is not justified because it appears to be the expression of a real relation of things, nor even because we have data at our disposal which could convince us that it does not and will never admit exceptions, that uncaused effects are impossible. All these things are lacking; but, we are told, they are not needed. The chief thing is that the causal law makes science possible, while to reject it means to reject science and knowledge generally, all anticipation, and even, as some few hold, reason itself. Clearly, if one has to choose between a slightly dubious admission on the one side and the prospect of chaos and insanity on the other, there will be no long hesitation. Apologetics, we see, has chosen the most powerful of arguments, _ad hominem._ But all such arguments partake of one common defect; they are not constant, and they are double-edged.