What is the theme of John Keats' "What the Thrush Said"?

To understand the themes running through this sonnet, one must go through the letter in which this piece had first appeared. Keats wrote this sonnet in a letter, dated February 18, 1818, to his friend John Hamilton Reynolds.

The first eight lines resounds with a highly optimistic note. The thrush promises that those have faced the severe winter, would certainly get to delight in the warm spring and those who have been suffering through darkness would...

To understand the themes running through this sonnet, one must go through the letter in which this piece had first appeared. Keats wrote this sonnet in a letter, dated February 18, 1818, to his friend John Hamilton Reynolds.

The first eight lines resounds with a highly optimistic note. The thrush promises that those have faced the severe winter, would certainly get to delight in the warm spring and those who have been suffering through darkness would surely get to see a morning three times brighter.

What follows in the following sestet, contains the main themes of the sonnet. It talks about being "passive and receptive" rather than handering after knowledge.

Earlier in the letter he says,

“Memory should not be called Knowledge - Many have original minds who do not think it - they are led away by Custom.

Keats proposes to be meditative and become patient observer. He says,

“How happy is such a voyage of concentration, what delicious diligent Indolence!”

This thought gets reflected in the following lines from this sonnet:

…He who saddens 
At thought of idleness cannot be idle, 
And he's awake who thinks himself asleep.

The poet says he would rather concentrate patiently and experience the knowledge that comes all by itself, as does a bee to a flower.

“let us not therefore go hurrying about and collecting honey, bee-like buzzing here and there impatiently from a knowledge of what is to be aimed at; but let us open our leaves like a flower and be passive and receptive.”

Thus, the main themes of this wonderful sonnet include the importance of being “passive and receptive” without being impatient.  If we be what the poet proposes, the thrush promises that the true knowledge and the real joy of experiencing it would follow as naturally as spring follows winter and day follows night.

O fret not after knowledge--