Some science behind the scenes
A sensitive is a person who is able to have spiritual experiences relatively easily. They are called sensitives because they are often not just more sensitive to the spiritual world but have higher sensitivity to pain and pleasure.
There are two medical theories as to why a person is a sensitive. The first theory is that a person has an unusual configuration of their brain [the hardware] caused by abnormal birth or inherited genes or premature birth or simple brain damage. The second theory is that the person does not produce inhibiting chemicals to the same degree as other people – again however, the reason why this occurs may be genetic or may be caused by an unusual brain configuration, so ultimately we are dealing with the same causes.
The Varieties of Psychedelic Experience – Dr Robert Masters and Dr Jean Houston
The theory often underlying … use of .. psycho chemicals is that ESP is a natural faculty of mind that is inhibited or kept inoperative by the production of body chemicals developed in the course of human evolution to serve just that inhibitory purpose. The inhibiting chemicals, that is, served to prevent man from perceiving a 'larger reality' that would be too distracting and so interfere with his survival and progress in the world. The 'medium' or 'sensitive', then, is a person whose body does not produce these chemicals in the normal amount and whose mind, therefore, is not normally inhibited. If, as has been argued, psychedelic drugs 'inhibit the inhibitors', then the drug subject might be able to experience what the medium experiences.