PROF. TAIT is not the only one who has to complain of hard treatment in the pamphlet by Mr. Herbert Spencer, referred to in the Professor's letter of last week. As the unlucky author of the obnoxious criticism that gave rise to the pamphlet in question, I of course come in for a lion's share of the abuse ; but neither Prof. Tait nor myself are, after all, treated so cruelly as is Newton, who, though his life was spent in maintaining the experimental character of all physical science, is cited as an authority for the à priori character of the most important of all physical truthsâ the well-known Three Laws of Motion.