plaigiarism?

Hmmm thought, if Freud sourced other places for his theories, without referencing then is this plagiarism? Of course there were different rules back in his day.
We all stand on the shoulders of others in regard to prior knowledge of course.
So I need to steer a course between a 'plot' and plagiarism.
I guess if knowledge is something that everyone is talking about at the time then it is common usage/ common knowledge really. There are some loose boundaries here of course.
Professor Kirsner's paper on Freud and stoicism looks like an interesting methodology, define each side then look for the parallels in Freud's work to the stoics. From a practical psychoanalytic point of view, reducing or masking your emotions is important, even whens some patients are clearly trying to shock you, or have a sadistic need to threaten or narcissistic need to be noticed. One requires enough empathy however, and it needs to be genuine, to be able to engage. Sounds more Aristotle, a balanced sense of one one ought.