Indefinite knowledge or incorrect interpretation can result from defective sensation. The defect may lie in the senses themselves, as a physiological fault in perception, or in the indefiniteness or blurring or contradiction in the sense stimuli, as found, for example, in optical illusions. The synthetic ability of the mind to piece together, or form a whole, from incomplete data often leads it to make errors. The mind sees what it is accustomed to seeing. Such errors in interpretation or imprecise knowledge may lead to incorrect judgements and as a result, to accidents. Unfortunately, when the habit of seeing what is desired to be seen instead of what is actually there is brought to bear on developmental and social issues, illusions may be maintained in official reports, particularly those showing satisfaction and giving positive evaluations for what is being done to address world problems.
The mind may substitute one identity for another, as in such cases as when an expected omnibus is seen although it is actually a truck. In other cases of illusion, the mind projects an identification where there is no object. For example the mind sees concrete images in abstract masses such as clouds, shadows, ink blots or tea leaves. In a third manner of illusion the mind compensates for what is there by seeing what ought to be there by custom, or by not seeing what is not there. Illusion by substitution, projection, and compensation is part of the mental process of perception.
An example in navigation is confused hearing of foghorns near high shores where echoes are thrown back, or in visual radar-control of fast-moving, apparently collision-bound aircraft, where rational after-images of blips can occur to the controllers.