Mereological essentialism is a metaphysical doctrine formulated and defended originally by Roderick M. Chisholm. The main principle 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). The principle may also be put by saying, that every object has the parts that it has necessarily. In the paper the above mentioned doctrine is reconstructed and analysed. Some consequences of the theory in question as well as many variants of it are examined. The main purpose of the text is to provide a critical analysis of merological essentialism in order to make this radical doctrine more acceptable.
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