"The spirit in Hamlet bears two names: Ghost and Spirit. Ghost is the personal name, whereas it is almost invariable referred to by the speakers as Spirit, and its statement begins with: I am thy Father's Spirit.
The word Spirit (in Latin Spiritus) also plays a great part in Bacon's scientific writings. The Spirit-theory, as will soon be briefly shown, is one of the principal points in Bacon's natural-philosophy. This spirit-doctrine is based on the views held by the natural-philosophers, Paracelsus, Telesius and Severinus Danus.
Theophrastus Paracelsus, the great Swiss thinker (he lived from 1493 to 1541) set up the theory; Bernadinus Telesius Consentinus, the Italian natural-philosopher (1508 - 1588) enlarged upon it and Petrus Severinus Danus, the Danish physician (died in 1602) reduced it to a distinct system. Bacon understood no German, or, at most, very little thereof. He can scarcely have studied the intellectually-rich and almost countless writings and pamphlets of Paracelsus as they were written in a style of German that was still clumsy and indistinct. But Bacon knew his theory from the principal work of Bernadinus Telesius of Cosenza, De Rerum Natura (Concerning the Nature of Things), of which the first two books appeared in 1565 and the whole was completed in 1586; he, furthermore, knew this theory thanks to the work of the Dane Petrus Severinus: Idea Medicinae Philosophicae (The Idea of a Philosophical Medical System), which work was written in clear and lucid Latin and serbed him (Bacon) as instructor in the science of healing, the work being based on natural science. When Bacon, in quite early youth, began to sketch out the plan of his Great Instauration - and we find traces of this aim as far back even as before his fourteenth year - the works of Telesius and Severinus were the newest in the field of natural-philosophy. Even Bacon himself, who very rarely mentions the names of other investigators, mentions the works above-named at short intervals in the 4th Chapter of the 3rd Book of his Encyclopedy: ("the Theory of Theophrastus Paracelsus, eloquently reduced into a body and harmony by Severinus the Dane; or that of Telesius of Consentium"), and he mentions two of them again (Bernadinus Telesius and Paracelsus) in the 3rd Chapter of the 4th ook, wherein he discusses the question of the human soul in detail.
But, as we shall soon see, the Spirit in Hamlet is not a being created at will by poetical imagination but clearly that personification of the natural-philosophical ideas of the spirit according to Bacon's views. And thus the views of Paracelsus accord with thos of Marcellus, while those of Bernadinus Telesius harmonise with those of Barnardo in the first act of Hamlet. And Hamlet himself represents the ideas of the third in the trio, namely, of the physician Severinus Danus (in English: the melancholy Dane). The time is out of joynt and Hamlet is born to set it right! He, like Severinus Danus, deals with comparative anatomy. Like Severinus Danus he is enamoured of that healing-art which is based on examination into natural laws."
- from The Shakespeare Secret by Edwin Bormann, 1895