Psak is mainly reasoning

 Igros Moshe (YD IV #38.2) Question: What is the significance of a sefer of collected rulings without explanation? Answer It would seem that if a scholar made a ruling in a specific case before him according to what he read in a book written by a great scholar which lacks the reasoning used for his ruling that that is not considered a halachic ruling at all and another authority can rule just the opposite whether strictltly or leniently. Nontheless it seems reasonable that in fact this is a halachic ruling since he is basing himself on the ruling of a well known authority who obviously had a legitimate reason for his ruling. Even though the second person did not know the reason it seems others can also rule like the original authority.