Murray, H. A. (1938). Explorations in Personality. New York: Oxford University Press.
We have selected the term press (plural press) to designate a directional tendency in an object or situation. Like a need, each press has a qualitative aspect -- the kind of effect which it has or might have upon the subject . . . as well as a quantitative aspect, since its power for harming or benefiting varies widely. Everything that can supposedly harm or benefit the well-being of an organism may be considered pressive, everything else inert. The process in the subject that recognizes that this is being done to him at the moment (that says 'this is good' or 'this is bad') may be conveniantly termed pressive preception. The process is definitely ego-centric, and gives rise almost invariably to some sort of adaptive behavior (Murray, 1938, pp. 118-119).

In identifying press we have found it convenient to distinguish between 1, the alpha press, which is the press that actually exists, as far as scientific inquiry can determine it, and 2, the beta press, which is the subject's own interpretatioin of the phenomena that he percieves. An object may, in truth, be very well disposed towards the subject -- press of Affiliation (alpha press) -- but the subject may misinterpret the object's conduct and believe that the object is trying to depreciate him -- press of Aggression: Belittlement (beta press). When there is wide divergence between alpha and beta press we speak of delusion (p. 122).