Less than 100 years before Yeats a philosopher as eminent as Kant could deem spiritualism worthy of serious treatment. Adorno later dealt with ans derided it. For Kant, the problem lay in its indigent conception of the sensible/ super-sensible relation. Like Kantian philosophy, spiritualism intuits a reality/ realm ‘beyond’ what can be sensuously apprehended. But whereas for Kant, this is a radically Other realm, inaccessible to our frames and categories, to the very grain of our language, for spiritualism it is a kind of hallucinatory, ethereal version of the empirically given. ‘Reality’ is subtended, supported by a kind of pallid simulacrum of itself. The ‘ghost’ of a person, for example, is merely a second body – hazy, transluscent, perhaps, lacking a usual relation to space and substance, but a kind of body nonetheless. Thus, for the spiritualist the so-called ‘beyond’ – the impossible ‘beyond’ which subtends space and time – is little more than the ‘here and now’ with its weight and volume subtracted. What such a conception fails to register is that we are dealing with two qualitatively different dimensions.
Thus, treated as a proposition (or set thereof) spiritualism is obviously false, in the manner of a category error. But one should be concerned, however, only with rescuing its ‘moment of truth’, which is to say: its affective charge, its existential and/ or poetic possibilities, its use-value. This, rather than its epistemological solidity is why people like Yeats were drawn to it... and I intend sposting something on this shortly, as part of an ongoing piece on Yeats, as it happens.
[J.Derbyshire has disputed whether Kant was indeed dealing 'seriously' with Swedenborg and spiritualism. 'Substantial treatment' may have been preferable, but in any case, the matter is disputed, see Gregory Johnson's intro and notes to this. Zizek (inevitably) also assigns a key place to this essay in Kant's philosophy (I can't remember where - perhaps Tarrying.)