The phrase "course of thought" refers to the path or direction that one's thinking takes. There are several synonyms for this phrase that can be used to convey the same basic meaning. For example, "train of thought" is a common substitution, as is "stream of consciousness." Other options include "line of thinking," "train of ideas," and "mental process." Each of these phrases conveys the idea that one's thoughts are following a specific path or trajectory, and they can be applied in a variety of contexts to describe the way in which one's mind is working or processing information.