So far as in that world it is a stable feature, holds ink, marks paper and obeys the guidance of a hand, it is a physical pen. [...] So far as it is instable, on the contrary, coming and going with the movements of my eyes, altering with what I call my fancy, continuous with subsequent experiences of its ‘having been’ (in the past tense), it is the percept of a pen in my mind.
1905, William James, “How Two Minds Can Know One Thing”, in Journal of Philosophy, Psychology and Scientific Methods