The Rhetoric of Immediacy: A Cultural Critique of Chan/Zen Buddhism By Bernard Faure p60 (I've not read this book) links it to linji ("the true man without affairs")
In his later years Dōgen often severely criticized the Senika heresy (sennigedo), which is the erroneous view that the mind abides while the form perishes. According to this view, there is a bright spiritual intelligence contained in our body that is the source of self-understanding. When the body dies, the spiritual intelligence alone does not perish but abides immutably. This view, Dōgen argues, when "hearing of the doctrine of this very mind (itself is buddha), takes it to mean that the discriminating knowledge of sentient beings is itself the buddha"
Dōgen (c. 1230-50) as cited in Masao Abe, Steven Heine (1992) A Study of Dōgen: His Philosophy and Religion. p. 158 The Senika heresy here mentioned is a Buddhist believe in eternalism of the self.
I'm interested, then, in whether it's only really a heresy in Soto Buddhism, and other traditions might find it useful.
I think it may be a useful fiction, which is why I ask.