As fast as thou shalt wane, so fast thou grow'st
In one of thine, from that which thou departest,
And that fresh blood which youngly thou bestow'st,
Thou mayst call thine when thou from youth convertest.
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'Youth rapidly wanes, but this decrease may be made up by the children that a young man may beget. Otherwise all are doomed to age and decay. Since you are manifestly so beautiful, let the fate of dying out be left to barren, harsh and sullen souls. You are so well endowed by nature, that it is clear she intended you to be a seal, from the impress of which many copies should be made'. shakespeares-sonnets.com
This takes up the same argument as sonnet I, and indeed it is a sonnet that can hardly stand on its own because 'if all were minded so' implies that we know already how the youth is minded to behave, information which we only derive from what has gone before.