It was on Descartes and on Hume that Huxley founded the precise form in which he urged the duty of doubt, and his exact words are worth quoting.

“It was in 1619, while meditating in solitary winter quarters, that Descartes (being about the same age as Hume when he wrote the Treatise on Human Nature) made that famous resolution, to “take nothing for truth without clear knowledge that it is such,” the great practical effect of which is the sanctification of doubt; the recognition that the profession of belief in propositions, of the truth of which there is no sufficient evidence, is immoral; the discrowning of authority as such; the repudiation of the confusion, beloved of sophists of all sorts, between free assent and merely piously gagged dissent, and the admission of the obligation to reconsider even one’s own axioms on due demand.”

This was the healthy and active scepticism which took no direct pleasure in doubting, but used doubt only as a means of making knowledge doubly secure, and which prevented false ideas being bolstered up by privilege or by tyranny.

“The development of exact natural knowledge in all its vast range, from physics to history and criticism, is the consequence of the working out, in this province, of the resolution to take nothing for truth without clear knowledge that it is such; to consider all beliefs open to criticism; to regard the value of authority as neither greater nor less than as much as it can prove itself to be worth.  The modern spirit is not the spirit ‘which always denies,’ delighting only in destruction; still less is it that which builds castles in the air rather than not construct; it is the spirit which works and will work ’without haste and without rest,’ gathering harvest after harvest of truth into its barns and devouring error with unquenchable fire.”

It is a special weakness of the modern human race to love inventing descriptive names by which particular modes of thought may be classified and labelled.  In order to meet this demand, Huxley invented the word agnosticism, to serve as a label for his own attitude.  The word rapidly became popular, and attempts were made to read into it far more than its inventor implied.  For him it was no definite body of doctrine, no creed in any positive sense.  It merely expressed the attitude he assumed towards all problems on which he regarded the evidence as insufficient.  It was a habit of mind rather than a series of opinions or beliefs; an intellectual weapon and not materials on which to exercise the intellect.