What does the line "mistress' eyes are nothing like the sun" literally mean?
This line literally means that his mistress' eyes are nothing like the sun. They are not made of the same material. They do not shine as bright; this can be taken metaphorically as well. But this is the point. Of course, her eyes are literally very different from the sun. Only a person with eyes that emanate heat and fire (literally) could be said to have eyes like the sun.
What does the line mean metaphorically? It was typical to compare a loved one's attributes with beautiful things in nature. But Shakespeare takes the opposite route here. He is respecting his beloved by being honest. Instead of making grandiose comparisons between the sun and her eyes, he is realistic. He ends the poem with the lines:
And yet, by heaven, I think my love as rare
As any she belied with false compare.
In other words, he thinks his love (his beloved) is as rare (strong, important, unique) as any other lover who's been compared with beautiful things in poetry. So, the line in question, literally and metaphorically, means the same thing. Her eyes are nothing like the sun. In the metaphorical negation, her eyes do not shine as brightly, they are not as radiant, and they are not as warm. The speaker wants to make the case that her eyes are warm and bright in their own unique way.
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