Final answer:
Breuer and Freud discovered the unconscious mind through the process of free association.
Step-by-step explanation:
The process that allowed Breuer and Freud to "discover" the unconscious mind was free association. Freud used various techniques such as dream analysis, slips of the tongue, and free association to access the unconscious mind. Through free association, patients would freely associate thoughts and feelings without censorship, which allowed unconscious thoughts and memories to emerge.