The phrase "thoughts of past" refers to memories or reflections of events that have already occurred. Synonyms for this phrase include reminiscences, recollections, musings, reflections, memories, and contemplations. Each of these words conveys a slightly different nuance in terms of the nature and tone of the thoughts being described. Reminiscences and recollections imply a nostalgic or sentimental tone, while musings and contemplations suggest a more introspective or philosophical mindset. Regardless of their specific connotations, these synonyms all describe a common human experience: the act of reflecting on one's past and processing the emotions and lessons learned from those experiences.