You dull your own perceptions
with false imaginings and do not grasp
what would be clear but for your preconceptions.
 
[Tu stesso ti fai grosso
col falso imaginar, sì che non vedi
ciò che vedresti se l’avessi scosso.]

Dante Alighieri (1265-1321) Italian poet

The Divine Comedy [Divina Commedia], Book 3 “Paradiso,” Canto 1, l. 88ff (1.88-90) [Beatrice] (1320) [tr. Ciardi (1970)]
    (Source)

Dante's beloved Beatrice greets him for the first time since his arrival in Paradise, chiding him for his terrestrial assumptions of what he's seeing.(Source (Italian)). Alternate translations:

False Forms deceive thy optics. Son of Man!
With shadowy objects which eclipse the true.
[tr. Boyd (1802), st. 20]

With false imagination thou thyself
Mak’st dull, so that thou seest not the thing,
Which thou hadst seen, had that been shaken off.
[tr. Cary (1814)]

Imagination false
Hath made thee dull, so that thou canst not see
That thou might'st, hadst thou looked diligently.
[tr. Bannerman (1850)]

Thou makest thyself so dull
With false imagining, that thou seest not
What thou wouldst see if thou hadst shaken it off.
[tr. Longfellow (1867)]

Thou thyself makest thyself gross with false imagining, so that thou seest not that which thou wouldest have seen, if thou hadst shaken it off.
[tr. Butler (1885)]

Thyself thou makest blind
With thy false fancy, that thou canst not see
What thou wouldst see, if this were thrown behind.
[tr. Minchin (1885)]

Thou thyself makest thyself dull with false imagining, so that thou seest not what thou wouldst see, if thou hadst shaken it off.
[tr. Norton (1892)]

Thou thyself makest thyself dense Earthly with false imagining, and so thou seest not what heavenly thou wouldst see, if thou hadst cast it off.
[tr. Wicksteed (1899)]

Thou dullest thine own wit
With false imagination, nor preceivest
That which thou wouldst perceive, being rid of it.
[tr. Sayers/Reynolds (1962)]

Thou makest thyself dull with false fancies so that thou canst not see as thou wouldst if thou hadst cast them off.
[tr. Sinclair (1939)]

Thou makest thyself dense of wit
With false fancy, so that thou dost not see
What thou would’st see, wert thou but rid of it.
[tr. Binyon (1943)]

You make yourself dull with false imagining, so that you do not see what you would see had you cast it off.
[tr. Singleton (1975)]

You are making yourself stupid
By imagining what isn’t, so that you do not
See what you would if you could shake that off.
[tr. Sisson (1981)]

You make yourself
obtuse with false imagining; you can
not see what you would see if you dispelled it.
[tr. Mandelbaum (1984)]

You have yourself to blame for burdening
your mind with misconceptions that prevent
from seeing clearly what you might have seen.
[tr. Musa (1984)]

You are making yourself swell
with false imagining, so that you do not see
what shaking it off would show.
[tr. Durling (2011)]

You make yourself stupid with false imaginings, and so you do not see, what you would see, if you discarded them.
[tr. Kline (2002)]

With false imaginings
you make yourself so dull you fail to see
what, shaking off this cloud, you’d see quite well.
[tr. Kirkpatrick (2007)]

You make yourself dull-witted
with false notions, so that you cannot see
what you would understand, had you but cast them off.
[tr. Hollander/Hollander (2007)]

You're overwhelming yourself with false
And foolish conjuring, preventing what your eyes
Would see if you did not struggle so hard for triumph.
[tr. Raffel (2010)]

You get all mixed up
By sticking with a figment of your imagination, so
You don’t see what you would see if you shook it off.
[tr. Bang (2021)]