... some spirits reside in more subtle matter, others in more dense matter; some reside in composite bodies, others in more simple bodies; some in observable bodies, others in unobservable bodies. As a result the operations of the soul are sometimes easier, sometimes more difficult, sometimes more weaker, sometimes well adapted, sometimes impossible. Some spirits operate within one genus, others act more efficaciously in another genus. Thus, humans possess certain operations and actions and desires not found in demons, and vice versa.
It is easy for demons to penetrate through bodies and to initiate thoughts in us. The reason for the latter is that they convey certain impressions directly to our internal senses, just as we ourselves sometimes seem to think of something suggested by the internal senses. This knowledge seems to occur according to the following comparison and analogy. If one wishes to generate a thought in someone standing at a distance, one must shout so that the thought is produced in their internal sense through their hearing it. But if the person is closer, a shout is not needed, only a quieter voice. And if a person is immediately nearby, a whisper in the ear suffices. But demons have no need for ears or voices or whispers because they penetrate into the internal sense directly, as we said. Thus, they send not only dreams and voices and visions to be heard and seen, but also certain thoughts which are hardly noticed by some. They communicate truths sometimes through enigmas, and sometimes through sense impressions. Sometimes they may even deceive. Not all things are granted to everyone, although they always happen in definite sequence and order.
Not all spirits and demons have the same level of existence, power and knowledge. Indeed we know that there are many more species of them then there are sensible things.
The role of the imagination is to receive images derived from the senses and to preserve, combine and divide them. This happens in two ways. First,it occurs by the free creative choice of the person who imagines, for example poets, painters, story writers and all who combine images in some organised way. Second, it occurs without such deliberate choice. The latter also happens in two ways: either through some other cause which chooses and selects, or through an external agent. The latter, again, is twofold. Sometimes the agent is mediated, as when a man uses sounds or appearances to bring about stimulations through the eyes or ears. And sometimes the agent is unmediated, as when a spirit, rational soul or demon acts on the imagination of someone asleep or awake, to produce internal images in such a way that something seems to have been apprehended by the external senses.
Giordano Bruno from 'On Magic' (1588) in Cause, Principle and Unity (Cambridge 1998)