Isaiah 26:3: Shakespeare on What to Think About

It matters on what or whom I think. This sonnet, this time, directed from me to God.

When to the sessions of sweet silent thought,
I summon up remembrance of things past,
I sigh the lack of many a thing I sought,
And with old woes new wail my dear time's waste:
Then can I drown an eye (unused to flow)
For precious friends hid in death's dateless night,
And weep afresh love's long since cancelled woe,
And moan th' expense of many a vanished sight.
Then can I grieve at grievances foregone,
And heavily from woe to woe tell o'er
The sad account of fore-bemoaned moan,
Which I new pay as if not paid before.
But if the while I think on thee (dear friend)
All losses are restored, and sorrows end.

Shakespeare's Sonnet #30.