Consider Figure 2 above on the left. After looking briefly at the field of letters, one notices the letter M nestled among the O s. What is harder to see is the letter Q because our eyes trick us into believing that all round letters in the field must be O s. The assumption that all round letters are O s limits our ability to perceive the difference between the letters at the bottom of the field. Limiting assumption can be very destructive.
Assumptions are the operating principle behind self-fulfilling prophecies.
If we are believe something to be true that is not tested or not true, we often acts as if it is true. A ssumptions must be tested and evaluated just as rigorously as evidence. Just as we weigh the merits of an author's claim, so too must we weigh the relevance and the validity of the author's assumptions.