Positive checks means Malthus’s term for measures and activities by which the life span of an existing human being is shortened in some way. Positive” is used here in a sense that the check is characterized by actions that terminate life—not in the colloquial sense that it is good or desirable. Malthus labels these "positive" checks because they actively cut down existing population by reducing the human life span. It is a most unfortunate choice of terms.
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