Sentence:  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 <WSD>percept</WSD> of a pen in my mind. 
Meaning of the word marked with <WSD> tags:  A perceived object as it exists in the mind of someone perceiving it; the mental impression that is the result of perceiving something. (from 19th c.)