Impurity

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Najaasaat (impurities) is the plural of najaasah (impurity), and it is everything which the people of sound nature hold to be dirty, and which they guard themselves from, and which they wash their clothes from if it fall upon them, such as urine and excrement.

And the basic principle for all things is permissibility (ibaahah) and that they are pure (at-Tahaarah). So whoever claims that a particular thing is impure must bring evidence. So if he produces it then that is the case; but if he is unable to do so, or he comes with something that does not establish proof, then what is obligatory upon us is to remain upon what is necessitated by the basic principle and the basic natural state.

This is because passing a judgement that something is impure is a ruling which will bring about general duty and responsibility, so it is not permissible except after proof has been established.