The projected form does make some impression upon the physical system. It is possible for it to be detected. It is a kind of pseudo-image, materialistically speaking, but it has definite electromagnetic reality and chemical properties. Animals have sensed such apparitions. They react to the chemical properties and build up [the perception] from these. These chemical properties are more diffused in such an apparition than in a physical form, however.

p. 338 - Seth, Dreams & Projections of Consciousness, (1986)