Heisenberg covers a broad range of topics from Greek philosophy through the development of Western philosophies regarding our understanding of the world around us.
The difficulty of metaphysical realism was felt soon after Descartes and became the starting point for the empiristic philosophy, for sensualism and positivism.
The three philosophers who can be taken as representatives for early empiristic philosophy are Locke, Berkeley and Hume. Locke holds, contrary to Descartes, that all knowledge is ultimately founded in experience. This experience may be sensation or perception of the operation of our own mind. Knowledge, so Locke states, is the perception of the agreement or disagreement of two ideas. The next step was taken by Berkeley. If actually all our knowledge is derived from perception, there is no meaning in the statement that the things really exist; because if the perception is given it cannot possibly make any difference whether the things exist or do not exist. Therefore, to be perceived is identical with existence. This line of argument then was extended to an extreme skepticism by Hume, who denied induction and causation and thereby arrived at a conclusion which if taken seriously would destroy the basis of all empirical science.
The criticism of metaphysical realism which has been expressed in empiristic philosophy is certainly justified in so far as it is a warning against the naïve use of the term, 'existence'. The positive statements of this philosophy can be criticized on similar lines. Our perceptions are not primarily bundles of color or sounds; what we perceive is already perceived as something, the accent here being on the word 'thing', and therefore it is doubtful whether we gain anything by taking the perceptions instead of the things as the ultimate elements of reality.
The underlying difficulty has been clearly recognized by modern positivism. This line of thought expresses criticism against the naïve use of certain terms like 'thing', 'perception', 'existence' by the general postulate that the question whether a given sentence has any meaning at all should always be thoroughly and critically examined. This postulate and its underlying attitude are derived from mathematical logic. The procedure of natural science is pictured as an attachment of symbols to the phenomena. The symbols can, as in mathematics, be combined according to certain rules, and in this way statements about the phenomena can be represented by combinations of symbols.
However, a combination of symbols that does not comply with the rules is not wrong but conveys no meaning.