[Footnote 22: It is well to remember that if common sense had said the last word about the matter, the Ptolemaic theory of the universe would still stand unshaken.] But, in addition to errors arising from the common intellectual nature of men, there are others, the sources of which are to be found in the idiosyncrasies of the individual mind. These Bacon calls Idols of the Cave;[23] for every one, he says, "has a cave or den of his own, which refracts and discolors the light of Nature, owing either to his own proper and peculiar nature; or to his education and conversation with others; or to his reading of books, and the authority of those whom he esteems and admires; or to the differences of impressions, accordingly as they take place in a mind preoccupied and predisposed, or in a mind indifferent and settled; and the like." This summary is comprehensive enough to indicate the character and point to some of the causes of individual aberrations of judgment; that it does no more than this is due to the simple fact that the personal bias is as varied as humanity itself, and that the deflecting impulses in any given case are to be referred to a complex of factors almost eluding analysis. To follow this part of the subject into detail would, therefore, manifestly be impossible. But certain of the larger and more widely influential of these disturbing forces may be roughly marked out by way of illustration. [Footnote 23: The metaphor is taken from the opening of the seventh book of Plato's Republic.] In the first place, there is what we may call the professional bias. Exclusive devotion to separate lines of activity, study, or thought inevitably gives the mind a particular set or twist. Bacon complains that Aristotle, primarily a logician, made his natural philosophy the slave of his logic. Few specialists can escape the insulation consequent upon living too continuously in a confined area of problems and ideas. Their intellectual outlook is necessarily circumscribed, facts are seen by them out of proper perspective, and one-sidedness of training and discipline renders their judgment of things partial and incomplete. The lawyer carries his legal, the theologian his theological, the scientist his scientific bent of mind into every inquiry; with what grotesque results is only too frequently apparent. Accustomed to move in a single narrow groove, and wholly absorbed in the contemplation of certain isolated classes of phenomena, they unconsciously allow their particular interests to dominate their thought, and impose disastrous restrictions upon their view of whatever lies outside their own chosen field.