When John Donne writes "Whatever dies was not mixed equally," he draws upon views of nature which had their origins in the ancient world by way of the Middle Ages.  The most important of these views is Galen's theory of the four humours.  According to Galen, the body was composed of liquids/fluids - yellow bile, black bile, phlegm, and blood - which regulated the workings of the body.  Illness, and by extension death, resulted from an imbalance in those humours.  For this reason, we have the term "phlegmatic" when we want to refer to someone who is apathetic and sluggish, or someone who is not motivated easily.  The idea of balance also extended to the natural world as a whole.  Borrowing from Aristotle's view of nature, perfection in nature was the result of a balance.  Whatever was not balanced, like the human body, did not enjoy health.

Within John Donne's poem, the line serves as an indication that the love between the poet and the woman are complementary.  The former completes the latter, achieving a balance.  In the last lines of the poem, Donne writes, "If our two loves be one, or, thou and I/ Love so alike that none do slacken, none can die.  Donne makes the provision that the loves have to be of the same degree, or a balance is not obtained.