‘A Memoir on the Appearance of Spectres or Phantoms occasioned by Disease, with Psychological Remarks. Read by Nicolai to the Royal Society of Berlin, on the 28th of February, 1799.’ Source: Journal of Natural Philosophy, Chemistry and the Arts 6 (1803), 161–79.
Philosophers divide the human being into body and mind, because the numerous and distinct observations we make on ourselves oblige us to consider man particularly, as well in respect to his corporeal as his mental functions. Other philosophers have supposed that this subject might be treated with greater perspicuity by considering man as composed of body, soul, and mind. There can be no doubt but that these, and even more divisions might be invented. Such philosophers, however, have by no means considered that arbitrary systematic divisions, do not constitute an investigation of nature, and that philosophy often becomes more uncertain the more precisely we endeavour to distinguish and separate what nature has closely united. Sub-divisions in speculation seem as necessary as fences in fields, both are in themselves unproductive, and the more they are multiplied and extended the greater is the diminution of the fertility.
For my part, I will confess, that I do not know where the corporeal essence in man ceases, or where the mental begins; though I admit of the distinction, because the extreme differences can be clearly perceived.