Motivation, reason, the impossible
Inspired by reading of David Hume's Enquiry concerning the principles of Morals (EPM) edited by Tom L. Beauchamp.1
"Hume is often interpreted as arguing that no value judgment--however extreme, obscene, or cruel--is reasonable or unreasonable, just as no value judgment is factual. This interpretation needs careful assessment. A passion is 'unreasonable' for Hume not because the passion is inappropriate, as we suggest today when we say, 'It was unreasonable of him to be angry', but because the passion is based on an erroneous judgment, as when we say, 'It is unreasonable to have a desire to do what is impossible'. For example, if I desire to see my dead grandfather at a restaurant tonight and this desire together with my peculiar belief that he will be there lead me to go to the restaurant, my desire is unreasonable because the judgment that he is alive and will be at the restaurant is unreasonable. Hume thinks that the judgment, not the desire, is unreasonable."(Ibid. 47)
It is not Hume's idea I will comment on here but I agree with it.
It is the idea of the commentator that desiring something impossible in unreasonable. His grandfather example seems fine though. Here are two counter-examples:
It is impossible to achieve a perfect society and not unreasonable to desire.
It is impossible to remove all suffering in the world and not unreasonable to desire.2
One way to defend his view is that the case is some continuum such as the perfectness of society or the amount of suffering in the view, the claim does not hold.
1This one: http://www.elounge.com/pages/ProductDetails.aspx?ProductIndeks=1891812
2I'm using possible in the same collateral sense that he is, not any strict logical sense.