Alexander Pope

Some praise at morning what they blame at night;
But always think the last opinion right.
A Muse by these is like a mistress us'd,
This hour she's idoliz'd, the next abus'd;
While their weak heads, like towns unfortified,
Twixt sense and nonsense daily change their side.
Ask them the cause; they're wiser still, they say;
And still tomorrow's wiser than today.

http://www.poetryfoundation.org/poem/174163