On the contrary, the necessity for a proper psychological and physiological training to the student of philosophy is denied, on the one hand, by the "pure metaphysicians," who attempt to base the theory of knowing upon supposed necessary and universal truths, and assert that scientific observation is impossible unless such truths are already known or implied: which, to those who are not "pure metaphysicians," seems very much as if one should say that the fall of a stone cannot be observed, unless the law of gravitation is already in the mind of the observer.
On the other hand, the Positivists, so far as they accept the teachings of their master, roundly assert, at any rate in words, that observation of the mind is a thing inherently impossible in itself, and that psychology is a chimera--a phantasm generated by the fermentation of the dregs of theology.

Nevertheless, if M.Comte had been asked what he meant by "physiologic cerebrale," except that which other people call "psychology;" and how he knew anything about the functions of the brain, except by that very "observation interieure," which he declares to be an absurdity--it seems probable that he would have found it hard to escape the admission, that, in vilipending psychology, he had been propounding solemn nonsense.
It is assuredly one of Hume's greatest merits that he clearly recognised the fact that philosophy is based upon psychology; and that the inquiry into the contents and the operations of the mind must be conducted upon the same principles as a physical investigation, if what he calls the "moral philosopher" would attain results of as firm and definite a character as those which reward the "natural philosopher."[14] The title of his first work, a "_Treatise of Human Nature, being an Attempt to introduce the Experimental method of Reasoning into Moral Subjects_," sufficiently indicates the point of view from which Hume regarded philosophical problems; and he tells us in the preface, that his object has been to promote the construction of a "science of man." "'Tis evident that all the sciences have a relation, greater or less, to human nature; and that, however wide any of them may seem to run from it, they still return back by one passage or another.
Even _Mathematics_, _Natural Philosophy_, and _Natural Religion_ are in some measure dependent on the science of MAN; since they lie under the cognizance of men, and are judged of by their powers and qualities.

'Tis impossible to tell what changes and improvements we might make in these sciences were we thoroughly acquainted with the extent and force of human understanding, and could explain the nature of the ideas we employ and of the operations we perform in our reasonings....

To me it seems evident that the essence of mind being equally unknown to us with that of external bodies, it must be equally impossible to form any notion of its powers and qualities otherwise than from careful and exact experiments, and the observation of those particular effects which result from its different circumstances and situations.

And though we must endeavour to render all our principles as universal as possible, by tracing up our experiments to the utmost, and explaining all effects from the simplest and fewest causes, 'tis still certain we cannot go beyond experience; and any hypothesis that pretends to discover the ultimate original qualities of human nature, ought at first to be rejected as presumptuous and chimerical....
"But if this impossibility of explaining ultimate principles should be esteemed a defect in the science of man, I will venture to affirm, that it is a defect common to it with all the sciences, and all the arts, in which we can employ ourselves, whether they be such as are cultivated in the schools of the philosophers, or practised in the shops of the meanest artisans.