Although people cannot define existence and do use the term with some looseness, yet it is possible to give an extensive definition by pointing to th… - C. D. Broad

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Although people cannot define existence and do use the term with some looseness, yet it is possible to give an extensive definition by pointing to the sorts of things that everyone believes to exist. It is still easier to point to the sort of entities that people agree in believing not to exist, and happily, this is really what concerns us most. People mean by saying that causation only applies to things that exist that it applies, if at all, to what can change; and they believe that, if anything can change, it is things like chairs and tables and minds, and not those like the propositions of Euclid and the multiplication table. What would be agreed then is that, if causal laws apply to anything it is to what can change in so far as it changes.