I was thinking along the lines of the obvious, such as when found tautological the form must have been valid but I'm sure there is a more in-depth explanation that I can't reach.
closed as unclear what you're asking by Keelan♦, Philip Klöcking♦, James Kingsbery, Nick R, Alexander S King Feb 5 '16 at 19:27
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There's nothing deeper going on here. An argument, "P proves Q" is valid iff it is true for all assignments P,Q iff P-> Q is a logical truth, also known as a tautology
This does invoke soundness/completeness.