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.

In Sonnet 147, William Shakespeare brings together two themes that have agitated him before: firstly the at the time fairly commonplace notion of love – and, more to the point, desire – as a disease that weakens the mind to the point of an irrational madness and afflicts the body in a similarly stark fashion, and secondly the ways in which his mistress deviates from the ordinarily praised ideal of beauty.

The sonnet therefore returns the series firmly and identifiably to the 'Dark Lady' and the effect she is having on our poet in an unequivocally physical manner, leaving behind the reflections on the soul of the previous sonnet and concerning itself once more with his lust for someone he knows – or at the very least declares – to be neither traditionally beautiful nor morally sound.

With his solemn, near pious, Sonnet 146, William Shakespeare for the first and only time speaks directly to his soul and entreats it to look after itself; to stop expending its energy on the pursuit of outward, physical adornments which are all doomed to swift decay – effectively starving and weakening itself whilst feeding and strengthening the gluttonous body that is only meant to house it and that will soon succumb to death – and to instead let go of material riches and with the 'return' from 'selling' them, 'purchase' something infinitely more valuable: eternal life in concord with, and on the terms ordained by, God.

The poem makes no mention, nor does it allude to or reference indirectly, any lover, mistress, or wife, nor love itself, or sex. This, too, makes it unique in the collection. As does its close alignment with a Christian notion of redemption through spiritual nurture at the expense of, and in preference to, physical or material gratification.