In "Contents of a Dead Man's Pocket," why doesn't Tom permit himself to look down?Jack Finney's "Contents of the Dead Man's Pocket"

The scene in which Finney writes that Tom does not permit
himself to look down illustrates his lack of foresight before going out onto the ledge. 
His thinking clouded by his overriding ambition, Tom has one thought:  Retrieve the
yellow sheet.  Therefore, he ventures out onto the ledge without considering any danger
or consequences:

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It was hard for him to understand that he
actually had to abandon it--it was ridiculous--and he bagan to
curse....

For many seconds he believed he was going to
abandon the yellow sheet, that there was nothing else to do....But just the same, and he
couldn't escape the thought, this and other independent projects, some already done and
others planned for the future, would gradually mark him out from the score of other
young men in his company....And he knew he was going out there is the darkness, after
the yellow sheet fifteen feet beyond his reach.....

To
simply go out and get his paper was an easy task--he could be back here with it in less
than two minutes--and he knew he was not deceiving himself....On a sudden impulse, he
got to his feet...In the back of his mind he knew he'd better hurry and get this over
with before he thought too much, and at the window he
didn't allow himself to
hesitate.

Once on the ledge,
fear "stirred in his stomach," but he he moves, "not allowing himself time to think. 
Tom knows that if he looks down, his terror may shake him, and he may lose his balance
and fall to his death.  In addition, Tom's refusal to allow himself to look down is
symbolic of his single focus on his ambitions.  For, it is not until he considers his
plight that he considers how meaningless his death would be--empty, like the contents of
his pockets.  It is not until he does look down that he realizes his love for his wife
Clare.