It is not the spell or ability itself. e.g., A spell that says “Counter a spell” cannot target itself. e.g., An ability of a permanent can target that permanent, because abilities and their sources are separate objects.
§ 355.9.c Core Rules PDF (officieel)
§ 355 2. Make relevant choices.
§ 355.9 A target is a valid choice if it meets all of the following requirements:
