The paper is an attempt to defend the Chisholm's metaphysical doctrine called mereological essentialism. The main thesis of mereological essentialism states that 'for any objects 'x' and 'y' - if 'x' is ever a part of 'y', then 'y' is necessarily such that 'x' is a part of 'y' ', i.e. that all parts of 'y' are essential to it ('y' has them at any time that 'y' exists). This radical theory gives a categorisation of all objects via 'entia per se' and 'entia successiva'. The paper contains a critical survey of the theory of mereological essentialism: it deals with some important arguments against it and answers as well. The author tries to show that in defence of the theory and the categorisation in question the applicability of the doctrine of mereological essentialism must be somehow limited. He suggests that the best way in doing so is to restrict theses of mereological essentialism either only to objects 'qua occurrents' or only to objects 'qua continuants' or only to persons.
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