"An explanation of cause is not a justification by reason"

This quote by C. S. Lewis is emphasizing the difference between an explanation of cause and a validation by reason. A description of cause is merely a description of why something happened, while a reason by factor is an attempt to show that something is ideal or wrong. A description of cause does not necessarily imply that the cause is right or wrong, it merely discusses why something happened. A justification by reason, on the other hand, is an effort to prove that something is best or wrong. It is an attempt to supply a logical argument for why something should or ought to not be done. This quote is highlighting that an explanation of cause is not the like a reason by reason, and that one must not be confused for the other.