Dissociation is that separation which, in the absence of a constant association, is permitted by the law of association of images. It is the consciousness of one thing, without the necessary simultaneous consciousness of the other.
…two ideas may be so little allied that one of them may be present to the consciousness in an image which does not contain the other at all; in this way we can imagine red without imagining blue, and vice versa; we can also imagine sound without melody, but not melody without sound. I call this kind of separation Dissociation.
Separation of Firstness, or Primal Separation, called Dissociation, consists in imagining one of the two separands without the other. It may be complete or incomplete.
The first mode of separating one constituent of the phaneron from another consists in imagining a state of things as to which the one may be affirmed while the other is denied. Thus we may imagine that there are brilliant colors while there are no sounds whatever, or the reverse. I called this mode of analysis dissociation.
In a paper published in the Proceedings of the American Academy of Arts and Sciences for 1867 May 14, I defined the three ways in which an idea can be mentally isolated from another. They [are] Dissociation, Prescission, and Discrimination. Dissociation consists in imagining a case into which the former idea enters without the other.