What does the speaker of en el jardín de los espejos quebrados, calibran catches a glimpse of his reflection mean when he says to call a man a beast one must see within his heart

It means that we must know the person beforehand, and not trust in the appearance.

Explanation:

Caliban is a character from the tempest and when he said that to call a man a beast one must-see within his heart, he was referring to prejudizing when we meet someone. We tend to judge others according to the appearance but we must know the heart of the people first to make a fair judgment.

"a" because he is appealing to logic as oppose to appealing to the audience's emotions.