He connected cause and effect relationships only to the motions or states of things and not to the things themselves. Hume went so far as to call into question the validity of the very concept of causality. For him, so-called cause and effect were only the result of our imagination. Causation was a psychological, subjective explanation of our experience of the constant conjunctions of the various actions which we observed. To Hume, there were no logically necessary connections between causes and effects.
Causes merely occur prior to and contiguous with effects.
Hume further said that there is no uniformity of nature across time or space. As with the idea of causality, any such conclusion on the part of people is the result of habit and not something which is empirically verifiable. In Hume's view, science can make only historical statements about what has happened and can make no predictions about subsequent events. Every new occurrence has the same probability as any other. Hume suggested, however, that people should proceed as a matter of practical requirement to act as though past experiences can somehow be relevant to what may occur in the future.