When the sense impression has been received and interpreted, the mental process is still incomplete; it is nearly always accompanied by some emotional reaction, i.e., our feelings—pleasure, disgust, shame, etc. are stirred at the same time. These too often affect our inferences and distort our interpretation of what we have seen. For example, in witnessing a street accident in which a pedestrian and a motor car are involved, our observation and our inferences may be affected by pity for the victim, or by sympathy with the driver of the car. The influence of emotion upon our inferences often takes the form of "making the wish father to the thought" i.e., we imagine that we have seen evidences of what we wished to see.
Such are the main sources of error in observation; and it should be remembered that everything said about seeing applies equally to bearing and all the other senses. Lastly, memory—the power that enables us to store up experience—is not always a safe guide. Most people tend to remember incidents attended with feelings of pleasure and warmth, and to repress the memory of those unpleasant incidents which sends a shiver down the spine. Distance often lends enchantment to the view. The passage of time frequently casts a halo about past events. Memory has a habit of exaggerating or minimising pleasant or unpleasant sensations. Memory, too, may play strange pranks. Sometimes a faulty or fictitious memory can create 'authority.'
We have so many relevant examples of the unsatisfactory way in which the mind often works. The tendency is for us to remember only those facts or instances which bear out a belief we already possess; we shrink from the special effort required to take account of negative evidence. Superstitious people will be ready to quote examples of fatalities occurring, say, after thirteen have sat down to table; they have forgotten, or have not troubled to remark, how often similar fatalities have followed the sitting down of twelve or fourteen; or the cases where thirteen have sat down to table and no fatality at all has ensued. This disposition to neglect negative evidence is one of the forms that the working of prejudice may take.
When any belief is popularly held, perhaps because it brings comfort or pleasure to its holders, every fresh circumstance is made to support and confirm it; and, although many strong evidences may seem to contradict it, people either shut their eyes to them or depreciate them or get rid of them in some other way, rather than sacrifice their cherished conviction. A man was once shown in a temple the votive tablets hung on the walls by people who had escaped the perils of shipwreck and was asked whether he was not then convinced that his scepticism regarding the power of the gods was ill-founded. His answer—and a very good one, too—was: "But where are the portraits of those who perished in spite of their vows?"
All superstitions are much the same in which the deluded observers note and remember the prophecies which are fulfilled but neglect or forget those which come to nothing, even though the latter may be much more common. Apart from the fact that people, especially ignorant people, do not relish having their cherished convictions upset, they are peculiarly prone to the error of paying more attention and giving greater weight to affirmatives than to negatives; whereas in trying to establish the truth of any proposition, they should give far more consideration to those instances that appear to point to the contrary.
If at the time of observation, or a short time subsequently, we are unable to distinguish what we have seen from the inferences made or the emotions aroused, how much more difficult it will be after some considerable interval has elapsed, during which perhaps we have lived through the experience again in our imagination, and made further inferences with further emotional reactions!
Unless we have taken care to make a careful record of our observations when they were still fresh, our memory may, quite unconsciously, distort or elaborate them. A witness's testimony in the law-courts is often a jumble of facts, assumptions and feelings, and a cross-examining counsel is usually not slow to take advantage of his inability to keep them separate, and thus to discredit him as a witness. In general, the tendency is for people to see what they want to see and to remember what they want to remember. Prejudice thus plays a large part in determining people's power of recall, and the scope and direction of their observation.