short story about alien being trapped in farmer's body

[cuddlyappliance]

Hi all,

I am trying to find the author of a story I read as a teen. It involved an alien being (gaseous if memory serves me correct) inhabiting the body of a fairly thick farmer or laborer. As the story progresses, the human realises he can understand more complex ideas every day and the alien being realises he is trapped in the human body. At the end, the human is saying he'll go to the UN with his advanced ideas and the alien is almost screaming that he's trapped.

Anyone know it? I have a feeling it's a Bradbury but I can't be sure.

Thanks