Truth and Error, published in 1899, and "treating of matter, motion, and consciousness as related to the external universe or the field of fact," as Gilbert describes it, and Good and Evil, running as a series of essays in the American Anthropologist, treating of the same factors as related to humanity or to welfare.

A third volume was planned to deal with the emotions, and he had also woven these ideas into a series of poems, of which only one has been published.

Few understand these later products of Powell.

Many condemn them; but Gilbert expresses his usual clear, unbiassed view of things and says (and I can do no better than to quote him, a man of remarkably direct thought, and for many years very close to Powell): "His philosophic writings belong to a field in which thought has ever found language inadequate, and are for the present, so far as may be judged from the reviews of Truth and Error, largely misunderstood.
Admitting myself to be of those who fail to understand much of his philosophy, I do not therefore condemn it as worthless, for in other fields of his thought events have proved that he was not visionary, but merely in advance of his time." One inexplicable action in his career, to my mind, was his complete ignoring in his report of the men and their work, of his second river expedition, particularly of his colleague, Prof.Thompson, whose skill and energy were so largely responsible for the scientific and practical success of the second expedition.

The report embodied all the results achieved by this expedition and gave no credit to the men who with unflagging zeal, under stress and difficulties innumerable accumulated the data.