The progress of physical science leads to the continual discovery of complexity in what is first apprehended as simple. The atom of hydrogen, so long accepted as the ideal limit of simplicity, is now suspected to be not the lowest unit in the scale of elements, and it is no longer conceived, as it used to be, as structureless, but as an individual system, comparable to a solar system, of electrical components preserving an equilibrium probably only temporary. The same tendency to discover complexity in what is first apprehended as simple is evident in the study of philosophy. The more our simple and ordinary notions are submitted to analysis, the more are profound problems brought to consciousness. It is impossible to think that we do not know what such an ordinary, simple notion as that of truth is; yet the attempt to give a definition of its meaning brings quite unexpected difficulties to light, and the widest divergence at the present time between rival principles of philosophical interpretation is in regard to a theory of the nature of truth. It is not a problem that is pressed on us by any felt need, nor is anyone who does not feel its interest called upon to occupy himself with it. We speak our language before we know its {10} grammar, and we reason just as well whether we have learnt the science of logic or not. This science of Logic, or, as it is sometimes called, of Formal Logic, was, until modern times, regarded as a quite simple account of the principles that govern the exercise of our reasoning faculty, and of the rules founded on those principles by following which truth was attained and false opinion or error avoided. It was called formal because it was supposed to have no relation to the matter of the subject reasoned about, but only to the form which the reasoning must take. A complete account of this formal science, as it was recognised and accepted for many ages, might easily have been set forth within the limits of a small volume such as this. But the development of modern philosophy has wrought an extraordinary change. Anyone now who will set himself the task of mastering all the problems that have been raised round the question of the nature of logical process, will find himself confronted with a vast library of special treatises, and involved in discussions that embrace the whole of philosophy. The special problem of truth that it is the object of this little volume to explain is a quite modern question. It has been raised within the present generation of philosophical writers, and is to-day, perhaps, the chief controversy in which philosophers are engaged. But although it is only in the last few years that controversy has been aroused on this question, the problem is not new--it is indeed as old as philosophy itself. In the fifth century before Christ, and in the generation that immediately preceded Socrates, a famous philosopher, Protagoras (481-411 B.C.) published a book with the title _The Truth_.