…when he wrote Il Penseroso:

Hence vain deluding Joys,
    The brood of Folly without father bred,
How little you bested,
    Or fill the fixed mind with all your toys;
Dwell in some idle brain,
    And fancies fond with gaudy shapes possess,
As thick and numberless
    As the gay motes that people the sunbeams,
Or likest hovering dreams,
    The fickle pensioners of Morpheus’ train.

…but also when he wrote L’Allegro:

Hence loathed Melancholy,
    Of Cerberus, and blackest Midnight born,
In Stygian cave forlorn,
    ‘Mongst horrid shapes, and shrieks, and sights unholy;
Find out some uncouth cell,
    Where brooding Darkness spreads his jealous wings,
And the night-raven sings;
    There under ebon shades, and low-brow’d rocks,
As ragged as thy locks,
    In dark Cimmerian desert ever dwell.

Or, in the words of another poet:

I know there’s a balance. I see it when I swing past.