Men should not be too smug in their own reason;
only a foolish man will walk his field
and count his ears too early in the season;
for I have seen a briar through winter’s snows
rattle its tough and menacing bare stems,
and then, in season, open its pale rose.
and I have seen a ship cross all the main,
true to its course and swift, and then go down
just as it entered its home port again.

[Non sien le genti, ancor, troppo sicure
a giudicar, sì come quei che stima
le biade in campo pria che sien mature;
ch’i’ ho veduto tutto ’l verno prima
lo prun mostrarsi rigido e feroce,
poscia portar la rosa in su la cima;
e legno vidi già dritto e veloce
correr lo mar per tutto suo cammino,
perire al fine a l’intrar de la foce.]

Dante Alighieri (1265-1321) Italian poet

The Divine Comedy [Divina Commedia], Book 3 “Paradiso,” Canto 13, l. 130ff (13.130-138) [Thomas Aquinas] (1320) [tr. Ciardi (1970)]
    (Source)

(Source (Italian)). Alternate translations:

Let none presume to fix his final state,
Or on such awful question hold debate;
Oft have I seen the vernal stem beguile
The reaper's hand: and oft the rigid thorn,
That to the blast of winter waves forlorn,
In June with rosy wreath is seen to smile.
Oft-times the bark that feuds with prosp'rous gale
Thro' the dividing waves with flowing sail.
Yet sinks in view of port, the pious man
May fail.
[tr. Boyd (1802), st. 23-24]

Let not the people be too swift to judge,
As one who reckons on the blades in field,
Or ere the crop be ripe. For I have seen
The thorn frown rudely all the winter long
And after bear the rose upon its top;
And bark, that all the way across the sea
Ran straight and speedy, perish at the last,
E’en in the haven’s mouth.
[tr. Cary (1814)]

Let not the people be too swift to judge,
Like one who looks upon the springing blade,
As if the harvest were already made.
For I have seen, the whole of winter long,
The thorn look rude and rough, and bare at top,
And after show the rose's reddening cup;
And seen the bark, already swift direct
Across the sea, in all its journey's way,
Perish at last when entering in the bay.
[tr. Bannerman (1850)]

Nor yet shall people be too confident
In judging, even as he is who doth count
The corn in field or ever it be ripe.
For I have seen all winter long the thorn
First show itself intractable and fierce,
And after bear the rose upon its top;
And I have seen a ship direct and swift
Run o'er the sea throughout its course entire,
To perish at the harbour's mouth at last.
[tr. Longfellow (1867)]

Let not the folk be yet too secure at judging, like him who values the corn in a field before it is ripe; for I have seen all winter long the plum-tree at first show itself rigid and stern, and afterward bear blossoms on its top ; and I saw on a time a craft trim and swift to sail the sea for its whole course, perish at the last in the entering of the sound.
[tr. Butler (1885)]

Let not the people think themselves elected
To judge like one who counteth on the corn
Within his field ere it be ripe.
Dejected I have beheld through winter time a thorn
Its rude repelling aspect show, and bear
After a rose, upon its top forlorn.
And I have seen a vessel swiftly steer
Through all its voyage across the ocean stream.
Perish at last, the harbour's entrance near.
[tr. Minchin (1885)]

Let not the people still be too secure in judgment, like him who reckons up the blades in the field ere they are ripe. For I have seen the briar first show itself stiff and wild all winter long, then bear the rose upon its top. And I have seen a bark ere now ran straight and swift across the sea through all its course, to perish at last at entrance of the harbor.
[tr. Norton (1892)]

Let not folk yet be too secure in judgment, as who should count the ears upon the field ere they be ripe;
for I have seen first all the winter through the thorn display itself hard and forbidding and then upon its summit bear the rose;
and I have seen ere now a ship fare straight and swift over the sea through her entire course, and perish at the last, entering the harbour mouth.
[tr. Wicksteed (1899)]

So also let not the people be too sure in judging, like those that reckon the corn in the field before it is ripe. For I have seen the briar first show harsh and rigid all through the winter and later bear the rose upon its top, and once I saw a ship that ran straight and swift over the sea through all its course perish at the last entering the harbour.
[tr. Sinclair (1939)]

Let not the people be too self-assured
In judging early, as who should count the rows
Of green blades in the field ere they matured.
For I have seen how first the wild-brier shows
Her sprays, all winter through, thorny and stark,
And then upon the topmost bears the rose;
And I have seen ere now a speeding barque
Run all her sea-course with unswerving stem
And close on harbour go down to the dark.
[tr. Binyon (1943)]

No one should ever be too self-assured
In judgement, like a farmer reckoning
His gains before the corn-crop is matured,
For I have seen the briar, a prickly thing
And tough the winter through, and on its tip
Bearing the very rose at close of spring;
And once I saw, her whole long ocean trip
Safe-done, a vessel wrecked upon the bar,
And down she went, that swift and stately ship.
[tr. Sayers/Reynolds (1962)]

Moreover, let not folk be too secure in judgment, like one who should count the ears in the field before they are ripe; for I have seen first, all winter through, the thorn display itself hard and stiff, and then upon its summit bear the rose. And I have seen ere now a ship fare straight and swift over the sea through all her course, and perish at the last as she entered the harbor.
[tr. Singleton (1975)]

Let people not be too sure of themselves
And their judgement, like someone who reckons
The field of corn before the ears are ripe:
For I have seen all the winter through
The thorn first show itself unyielding, wild,
And after all carry a rose on top;
And I have seen a ship sail straight and swiftly
Over the sea for the whole of its voyage
Yet perish at last at the harbour mouth.
[tr. Sisson (1981)]

So too, let men not be too confident
in judging -- witness those who, in the field,
would count the ears before the corn is ripe;
for I have seen, all winter through, the brier
display itself a stiff and obstinate,
and later, on its summit, bear the rose;
and once I saw a ship sail straight and swift
through all its voyaging across the sea,
then perish at the end, at harbor entry.
[tr. Mandelbaum (1984)]

Nor should one be too quick to trust his judgment;
be not like him who walks his field and counts
the ears of corn before the time is ripe,
for I have seen brier all winter long
showing its rough and prickly stem, and then
eventually produce a lovely rose,
and I have seen a ship sail straight and swift
over the sea through all its course, and then
about to enter in the harbor, sink.
[tr. Musa (1984)]

And let not people be too sure to judge, like one who appraises the oats in the field before they are ripe:
for I have seen all the previous winter long the thornbush appear rigid and and fierce, but later bear the rose upon its tip,
and I have seen a ship run straight and swift across the sea for all in its course, only to perish at last when entering the port.
[tr. Durling (2011)]

Do not let people be too secure in their judgements, like those who count the ears of corn in the field before the crop ripens, since I have seen, all winter long, the thorn display itself, sharp and forbidding, and then on its summit bear the rose; and before now I have seen a ship run straight and sure over the sea for her entire course, and sink in the end, entering the harbour mouth.
[tr. Kline (2002)]

And then again, don't let folk be too sure
in passing judgement as do those who price
the harvest in the field before it's ripe.
For I have seen, at first, all winter through
a thorn bush shows itself as stark and fierce,
which after bears a rose upon its height.
And I have seen a keel, steered swift and well,
speed over oceans all its voyage through,
then perish at the entrance to the dock.
[tr. Kirkpatrick (2007)]

Let the people, then, not be too certain
in their judgments, like those that harvest in their minds
corn still in the field before it ripens.
For I have seen the briar first look dry and thorny
right through all the winter's cold,
then later wear the bloom of roses at its tip,
and once I saw a ship, which had sailed straight
and swift upon the sea through all its voyage,
sinking at the end as it made its way to port.
[tr. Hollander/Hollander (2007)]

But ordinary people, too, must guard
Their judgment, not like those who count up ears
Of corn before the field is ripe. For I
Have seen, all winter through, bushes of thorn
Covered with small but savage knives, hard
And fierce, but now comes summer, and they they're roses
All over. And I have seen a ship sail far,
Straight and swift, and on course, but once in the harbor
Down she goes, sinking like a stone.
[tr. Raffel (2010)]