thinking is a sickness of the eyes

Pessoa understood that sensations themselves are full of contradiction. The invention of Caeiro allowed Pessoa to embody that inherent contradiction and to mine it for all its interesting conflicts while writing poetry that allowed him, through the simple pose of a shepherd-poet, to act unaware or unconscious of it. In perfect illustration of this, Pessoa wrote of Caeiro, “In fact, he never contradicts himself, and when it seems that he does, there appears, in some other corner of his verse, the appropriate contrary argument.” In other words, Caeiro never contradicts himself, and when it looks as though he does, don't worry: there’s another contradiction somewhere else contradicting this contradiction.

Here.