In Sonnet 148, William Shakespeare develops the themes revisited with Sonnet 147 and further elaborates on his realisation that reason has abandoned him and he is therefore incapable of judging properly what he sees. Either that, or his eyes themselves are faulty, since they seem to distort what they are looking at.

The conclusion he comes to, much in line with the previous sonnet, is that his defective vision stems from his love for his mistress, but he here adds the almost 'technical' but for this not at all inconsequential detail that his eyes couldn't possibly be expected to deliver a true picture to the brain of what they see, since their vision is blurred by tears, suggesting therefore that this love he feels for his mistress is tinged with sadness, sorrow, or pain.