David Hume (1711-76) is the culminating genius of the Empiricist tradition, soon undermined even this claim. After so many occurrences of seeing the same image, the observer begins to assume that all of these temporarily distinct yet visually identical images are really the same thing, and thus naturally jumps to the conclusion that they must all be caused by one unchanging object in the room.
The belief in the existence of an outer world depends on the concept of cause and effect: that is, one assumes the sense impressions received by the mind are caused by objects out in the world. He examined several instances of supposed causal relationship, between fire and heat, for example, but in every instance he pointed out that the sensory evidence itself displays only one event, and then another close by, but no necessary connection. All events seem entirely loose and separate. One event follows another: but we never can observe any tie between them. They seem conjoined, but never connected. The necessary conclusion seems to be that we have no idea of connexion or power at all, and that these words are absolutely without any meaning, when employed either in philosophical reasoning or common life.
Many of the other ideas used to justify a belief in an objective outer world, like the ideas of uniformity in nature, the existence of other minds, Hume demolished with a similar line of reasoning. All the mind can ever gain from the senses themselves, Hume showed time and time again, is a random flux or intermittent and constantly changing impressions.
Although Hume demolished the various inferences made by the mind from its sense impression about a so-called objective world, he admitted that all minds seem to have an irresistible urge to make such leaps.
The mind is not content to see only a random array of different images; instead, it actively searches for the features which seem common to all the images, conveniently ignores the differences, and even fills in the gaps when the table is not being perceived. The mind’s desire to see order in variable and interrupted impression is so great that it acts and transforms some similarity in images to complete similarity, or identity. To resolve the contradiction between what it sees and what it wants to see, the mind invents an idea of continuous substance which it supposes lies behind all the interrupted images. Thus we feign the continued existence of the perceptions of our senses, to remove the interruption; and run into the notion of a soul, and self, and substance, to disguise the variation.
With Hume a tree hundred year old philosophical program had come to an inevitable and unsatisfactory end. The concepts of mind and world which held so much promise when they were first formulated in the early Renaissance were now seen to be problematical in the extreme, leading either to empty metaphysical fantasies in its Rationalist mode or to random impressions and a lack of certainty in the Empiricist mode. In both cases the logic and the lines of reasoning leading to the unsatisfactory conclusions were above reproach. By 1750 the philosophers had reached a crisis in their understanding of the mind and the world, a crisis which would lead in the next generation to a major reorientation of the West’s philosophical assumptions.