Knuth's analogy to an organism that looks at a .tex input line by line with its eyes, and during pre-processing (transcoding to ASCII/Unicode, removal of trailing paces, appending endlinechar) takes the line as a directive to put characters in its mouth and to chew those characters into tokens, and to swallow the tokens inton the gullet.
In doing so, drawing attention to the difference between "character" and "character token". In the context of tokenizing, also addressing category codes, and in what ways tokens can be classified ((By state: macro/primitive/undefined/\chardef-token/implici…