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An indirect proof is a proof wherein you begin by assuming temporarily that the desired conclusion is not true which then by reasoning logically reaches to a certain contradiction or some other known fact.

1. Assume temporarily that the conclusion is not true.

2. Reason logically until you reach a contradiction.

3. Point out that the assumption was wrong and the conclusion must then be true.

The first sentence of his indirect proof was 鈥淎ssume temporarily that $l$and $m$are intersecting lines.

Skew lines are lines that do not intersect and are not parallel.

They lie in different planes.

Start off an indirect proof by assuming temporarily that the given conclusion is incorrect. In this case, the correct sentence would be:

鈥淎ssume temporarily that $l$and $m$are not skew line鈥�.

This covers the entire set of lines that are not skewed, which would be parallel lines,

Perpendicular lines and intersecting lines.

When John said 鈥淎ssume temporarily that $l$ and $m$ are intersecting lines鈥�, this does not cover parallel lines and perpendicular line.