What does the speaker of "En el Jardín de los Espejos Quebrados" mean when he says "To call a man a beast, one must see within his heart"?

In this case, the speaker is Caliban. With this sentence, Caliban tries to say that we should get to know someone before we start judging without knowing anything about someone. This refers to the prejudices people often have about other people without knowing anything about them.

Therefore, Caliban tries to say that we should meet someone, come closer to his/her hearts before bringing any conclusions. People's appearance doesn't matter; we should become familiar with their soul and their hearts so we could make a fair judgment.