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What is Loss of capability of doing goal-directed things, to express ideas, making irrelevant comments, but ultimately succeeding in going back to the original point is known as - and explain in detail?
Both circumstantiality and tangentiality have degression into unnecessary irrelevant details. However:-


In circumstantiality, thought reaches its ultimate goal in a long and roundabout manner, i.e. the patient return back to the original theme after degression.
In tangentiality, the goal is not reached and the thought is sidetracked more and more away from the natural end and never returns to the original theme.