But many do not know that they do not know, and many think they know when they know nothing. Failings of the intelligence are incorrigible since those who do not know do not know themselves and cannot therefore seek what they lack.

[Pero hay muchos que ignoran que no saben y otros que piensan que saben, no sabiendo. Achaques de necedad son irremediables, que como los ignorantes no se conocen, tampoco buscan lo que les falta.]

Baltasar Gracián y Morales (1601-1658) Spanish Jesuit priest, writer, philosopher

The Art of Worldly Wisdom [Oráculo Manual y Arte de Prudencia], § 176 (1647) [tr. Jacobs (1892)]
    (Source)

(Source (Spanish)). Alternate translations:

Yet there are some, who are ignorant that they know nothing; and others, who think they know, though they know nothing at all. The faults that proceed from the want of wit, are incurable. For as ignorants know not themselves, so they take no care to search for that they want.
[Flesher ed. (1685)]

But there be many who do not know that they know nothing, and others who think that they know, but know nothing; these deformities of the mind are incurable, whence it is that the ignorant neither know themselves nor yet how to gain what they lack.
[tr. Fischer (1937)]

But many people are unaware that they do not know, and others think they know when they do not. Attacks of foolishness have no remedy. Because the ignorant do not know themselves, they never look for what they're lacking.
[tr. Maurer (1992)]