Fear is a general feeling of anxiety and agitation caused by the presence of nearby danger, evil, or pain. Uncertainty is the lack of definite knowledge, doubt, and the unknown. as a theme in A Boys Will.
'The Demiurge's Laugh' is one example. Plato's Demiurge was a creature ordered to the design of humans. When his creations were lower than the ideal form the Demiurge was indifferent to this and was often pessimistic of their aspirations. Another myth closely related to this story was of King Midas. It was said that the King once hunted the wise daemon Silenus. When he finally caught him, Silenus remained silent, but then broke into laughter and responded to his captor with indifference and negativity (Thompson 326-327). Thompson states 'Frost created a parable based on the raw material of these old myths.' (327) That I suddenly heard ' all I needed to hear:
It lasted me many and many a year.
The sound was behind me instead of before,
A sleepy sound, but mocking half,
As of one who utterly couldn't care.
The Demon arose from his wallow to laugh,
Brushing the dirt from his eyes as he went;
And well I knew what the Demon meant.

'He represented himself as having conducted a search for the modern Demiurge named Evolution in hope of learning the secrets of life, but when finally found him all he was rewarded was indifference, atheism, and laughter' (Thompson 327). The uncertainty lies in the Demiurge's answer of indifference and atheism. This answer may inspire some fear but much of it resides in the face of Evolution, an element of nature.
Another poem that conveys fear and uncertainty is 'Into My Own.' In lines 1 through 4 Frost writes:

One of my wishes is that those dark trees,
So old and firm they scarcely show the breeze,
Were not, as 'twere, the merest mask of gloom,
But stretched away unto the edge of doom.

The dark trees are scary enough but to enhance things they wear a mask of gloom. This is clearly an example of fear. The trees being stretched away unto the edge of doom shows the uncertainty and unknown. Booth states 'Robert Frost's deeds have been the words of dark poems, narrative and lyric: poems that explore those conflicts which, as they are partially resolved by definitions, allow a man to range further and further into the unknown' (1: 213).
Uncertainty is showed as a theme in the poem 'Mowing.' Lines 1 to 6:

There was never a sound beside the wood but one,
And that was my long scythe whispering to the ground.
What was is whispered? I knew not well myself;
Perhaps it was something about the heat of the sun,
Something, perhaps, about the lack of sound'
And that was why it whispered and did not speak.

Frost shows uncertainty in what the speaker's scythe is saying. It is not known what was whispered but the speaker knows something was said. Greiner states ''man-made order confronting uncaring nature'' (54: 101). This is also an example of uncertainty, maybe even fear in the face of the surrounding nature.
A classic example of the use of fear is in the poem 'Storm Fear.' In it, Frost gives fearful qualities to an element of nature: storms. Lines 1 to 7:

When the wind works against us in the dark,
And ...