--Roger Bacon (c.1210 - c.1292)
    "On the Usefulness of Grammar", chapter I 
    Opus Majus (c.1268)

nam nihil aegrius est quam res secernere apertas
ab dubiis, animus quas ab se protinus addit.

Denique nil sciri siquis putat, id quoque nescit
an sciri possit, quoniam nil scire fatetur.
hunc igitur contra minuam contendere causam,
qui capite ipse suo in statuit vestigia sese.
et tamen hoc quoque uti concedam scire, at id ipsum
quaeram, cum in rebus veri nil viderit ante,
unde sciat quid sit scire et nescire vicissim,
notitiam veri quae res falsique crearit
et dubium certo quae res differre probarit.
invenies primis ab sensibus esse creatam
notitiem veri neque sensus posse refelli.
nam maiore fide debet reperirier illud,
sponte sua veris quod possit vincere falsa.
quid maiore fide porro quam sensus haberi
debet? an ab sensu falso ratio orta valebit
dicere eos contra, quae tota ab sensibus orta est?
qui nisi sunt veri, ratio quoque falsa fit omnis.

Translated (c. 1675) by Lucy Hutchinson (1620-1681)

For tis most noble to discerne betweene
Reall, and dubious, even by thoughts unseene.

Now if some say, that nothing's to be knowne,
They know not that, who gen'rall ignorance owne.
Wherefore I shall not here strive to confute
Those who on falsehood ground their whole dispute.
But if this knowledge should be granted, yett
I aske, since things no certeintie admitt
From whence then doe their owne discoveries flow,
Which shew what tis to know, or not to know?
For you will find, men from their sence derive
That knowledge which doth just distinctions give
What's certeine, or uncerteine, false, or true:
And none can contradict what sensce can shew.
Wee can trust nothing with more confidence
Then what doth, of it selfe, by truth, convince
Things which are false; what then shall be believ'd
Before the sence? shall that which is receiv'd
By a false sence, those reasons overthrow
Which wholly doe from the true sences flow?
All reason's false, unlesse a certeintie
Be in the sence....

Translated (1864) by Hugh Andrew Johnstone Munro (1819-1885)

... For nothing is harder than to separate manifest facts from doubtful which straightway the mind adds on of itself.

Again if a man believe that nothing is known, he knows not whether this even can be known, since he admits he knows nothing.  I will therefore decline to argue the case against him who places himself with head where his feet should be.  And yet granting that he knows this, I would still put this question, since he has never yet seen any truth in things, whence he knows what knowing and not knowing severally are, and what it is that has produced the knowledge of the true and the false and what has proved the doubtful to differ from the certain.  You will find that from the senses first has proceeded the knowledge of the true and that the senses cannot be refuted.  For that which is of itself to be able to refute things false by true things must from the nature of the case be proved to have the higher certainty.  Well then what must fairly be accounted of higher certainty than Sense?  Shall reason founded on false sense be able to contradict them, wholly founded as it is on the senses?  And if they are not true, then all reason as well is rendered false.

Translated (1910) by Cyril Bailey (1871-1957)

For nothing is harder than to distinguish things manifest from things uncertain, which the mind straightway adds of itself.

Again, if any one thinks that nothing is known, he knows not whether that can be known either, since he admits that he knows nothing.  Against him then I will refrain from joining issue, who plants himself with his head in the place of his feet.  And yet were I to grant that he knows this too, yet I would ask this one question; since he has never before seen any truth in things, whence does he know what is knowing, and not knowing each in turn, what thing has begotten the concept of the true and the false, what thing has proved that the doubtful differs from the certain?  You will find that the concept of the true is begotten first from the senses, and that the senses cannot be gainsaid.  For something must be found with greater surety, which can of its own authority refute the false by the true.  Next then, what must be held to be of greater surety than sense?  Will reason, sprung from false sensation, avail to speak against the senses, when it is wholly sprung from the senses?  For unless they are true, all reason too becomes false.

translated (1916) by William Ellery Leonard (1876-1944)

For naught is harder than to separate
Plain facts from dubious, which the mind forthwith
Adds by itself.

                      Again, if one suppose
That naught is known, he knows not whether this
Itself is able to be known, since he
Confesses naught to know.  Therefore with him
I waive discussion--who has set his head
Even where his feet should be.  But let me grant
That this he knows,--I question:  whence he knows
What 'tis to know and not-to-know in turn,
And what created concept of the truth,
And what device has proved the dubious
To differ from the certain?--since in things
He's heretofore seen naught of true.  Thou'lt find
That from the senses first hath been create
Concept of truth, nor can the senses be
Rebutted.  For criterion must be found
Worthy of greater trust, which shall defeat
Through own authority the false by true;
What, then, than these our senses must there be
Worthy a greater trust?  Shall reason, sprung
From some false sense, prevail to contradict
Those senses, sprung as reason wholly is
From out of the senses?--For lest these be true,
All reason also then is falsified.

Translated (1951) by Ronald Edward Latham (1907-1992)

There is nothing harder than to separate the facts as revealed from the questionable interpretations promptly imposed on them by the mind.

If anyone thinks that nothing can be known, he does not know whether even this can be known, since he admits that he knows nothing.  Against such an adversary, therefore, who deliberately stands on his head, I will not trouble to argue my case.  And yet, if I were to grant that he possessed this knowledge, I might ask several pertinent questions.  Since he has had no experience of truth, how does he know the difference between knowledge and ignorance?  What has originated the concept of truth and faleshood?  Where is his proof that doubt is not the same as certainty?

You will find, in fact, that the concept of truth was originated by the senses and that the senses cannot be rebutted.  The testimony that we must accept as more trustworthy is that which can spontaneously overcome falsehood with truth.  What then are we to pronounce more trustworthy than the senses?  Can reason derived from the deceitful senses be invoked to contradict them, when it is itself wholly derived from the senses?  If they are not true, then reason in its entirety is equally false.

Translated (1954) by Henri Clouard (1885-1973)

Rien n'est plus difficile en effet que de faire le départ entre la vérité des choses et les conjectures qu l'esprit y ajoute de son propre fonds.

Certains penseurs estiment que toute science est impossible; or ceux-là ignorent également si toute science est possible, puisqu'ils proclament ne rien savoir.  Je n'accepte point de débat avec quiconque prétend marcher la tête en bas.  Et quand bien même j'accorderais à ces gens quassurément l'on ne sait rien, je leur demanderais comment, n'ayant jamais trouvé la vérité, ils savent ce qu'est savoir et ne pas savoir, d'où ils tirent la notion du vrai et du faux et par quelle méthode ils distinguent le certain de l'incertain.

Tu verras que les sens sont les premiers à nous avoir donné la notion du vrai et qui'ls ne peuvent être convaincus d'erreur.  Car le plus haut degré de confiance doit aller à ce qui a le pouvoir de faire triompher le vrai du faux.  Or quel témoignage a plus de valeur que celui des sens?  Dira-t-on que si'ls nous trompent, c'est la raison qui aura mission de les contredire, elle qui est sortie d'eux tout enitière?  Nous trompent-ils, alors la raison tout enitière est mensonge.

Translated (1955) Alban Dewes Winspear (1899-1973)

The hardest task of mind is just to separate
The open, clear and certain things,
From dubious apparitions which the mind supplies itself.
Again, suppose a thoughtful man puts up this argument:
The human mind can nothing ever know.
This judgment, too, must turn out fallible.
For he admits the limitations of the human mind;
Nothing mind can know.
With such a man I'll never join in argument.
He firmly plants his head in footprints of his foot.
And yet were I to grant that he knows this--
Namely that he knows nothing certainly,
One question I would ask:--
Since he has never yet found truth in clear perception of a thing,
How can he know what knowledge means?
What ignorance, in turn?
What then has made to grow in him
The notion of the true; the false?
And what has made him come to think
That doubtful notion differs from the sure?
But if you really face the facts of things,
This you will find:--
That the very notion of the true emerges from experience of the sense;
That sense and sense impressions cannot be gainsaid.
For something must be found of greater certainty than sense
Which of itself can use the true to overthrow the false.

Translated (1963) by René Acuña (1929 -   )

Porque no hay nada más difícil, que distinguir los datos reales, de las cosas dudosas que el ánimo pone antes de darnos cuenta completa.

Para terminar, si alguno piensa que nada sabe, en consecuencia ignora la razón por que dice que nada se sabe.  Pasaré por alto, pues, el entablar discusión con este que, en las bases de su doctrina, da las armas contra sí mismo.  Y, sin embargo, aun concediéndole que sabe que no sabe, me gustaría preguntarle cómo sabe, si nunca ha visto en las cosas nada de verdad, lo que es saber o no saber; qué hecho ha originado la noción de lo verdadero y de lo falso, y qué criterio acostumbra para distinguir lo cierto de lo dudoso.

Se encontrará con que la noción de lo verdadero ha nacido de los sentidos principalmente, y que los sentidos no pueden ser desmentidos.  En efecto, debe gozar de mayor fe aquello que puede vencer los datos falsos con los hechos verdaderos.  Y para eso, ¿quién es más digno de le que los sentidos?  ¿En qué sentido falso apoyada podrá rechazarlos la razón, que nace toda ella de los sentidos?  Porque, si los sentidos no son verdaderos, el juicio lógicamente es falso.

Translated (1967) by Rolfe Humphries (1894-1969)

                                 ....Humankind
Finds nothing harder than to separate
The patent facts from those dubieties
Mind loves to introduce.

                                   But if a man
Argues that, therefore, nothing can be known,
He does not really even know that much
Since he's confessing total ignorance.
I'd best not argue with this kind of man
Who sticks his head in the ground, his feet in the air.
Still, let me grant he knows this much, I'll ask
How, since he's never caught one glimpse of truth
In anything whatever, how does he know
What knowing and non-knowing are, what fact
Gave him the notion of the true and false,
Assured him of a difference between
The doubtful and the certain?  You will find
All knowledge of the truth originates
Out of the senses, and the senses are
Quite irrefutable.  Find if you can,
A standard more acceptable than sense
To sort out truth from falsehood.  What can be
More credible than sense?  Shall reasoning,
Born of some error, some delusionment,
Argue the senses down?  Ridiculous!
If sense is false, reason will have to be.

Translated (1976) by Charles Hubert Sisson (1914-2003)

Nothing is harder than to distinguish between
Patent facts and the doubts which the mind contributes.

Anyone who thinks that nothing is known
On his own admission cannot assert even that.
I do not think it worth arguing a case
Against a man who is standing on his head.
But let us suppose he does know nothing is known;
I would ask him, since he has seen no truth in anything,
How he can know what knowing and not knowing are?
What are the marks of truth and the marks of falsehood?
How can one tell the doubtful from the certain?
You will find that it is the senses which first created
The criterion of truth, they cannot be shown to be wrong:
Credibility must attach to whatever
Makes it possible to set aside the false for the true.
What has more credibility than the senses?
Is it possible that a reason based on lying senses
Should contradict them, since it is them she comes from?
If they are not true, then reason itself is a lie.

Translated (1995) by Anthony Michael Esolen

Nothing's as tough as to set things in plain sight,
Apart from the instant error of the mind.

And whoever thinks that knowledge can't be had
Can't know that either--as he says, he knows nothing!
So against this man I'll cease to plead my case--
Let him stand on his head if that's his wish.
But let's suppose he knows this; still I'll ask,
Seeing that truth could never be found in things
How he knows what it is to know or not to know,
What gives him a sign of the true and false, what tests
And separates the doubtful from the sure?
You'll find that truth's criterion first proceeds
From the senses--which can never be proved false.
For something more worthy of trust would have to be found
Which of its own subdues the false with truth.
What then should be held more worthy of trust than the senses?
Or can reason arisen from false senses speak
Against them, arisen wholly from those senses?
If they're not true, all reason must be wrong!