In the beginning of algebra, even the most intelligent child finds, as a rule, very great difficulty. The use of letters is a mystery, which seems to have no purpose except mystification. It is almost impossible, at first, not to think that every letter stands for some particular number, if only the teacher would reveal what number it stands for.
Mysticism and Logic: And Other Essays, Chapter IV (p. 63), Longmans, Green & Co. 1919