Observation or experience rather than theory or pure logic.
Answer:
the Constitution explicitly assigns the president the power to sign or veto legislation, command the armed forces, ask for the written opinion of their Cabinet, convene or adjourn Congress, grant reprieves and pardons, and receive ambassadors.
another Answer:
Congress shares with the president the power to make treaties and to appoint ambassadors. As for the president himself, he is assigned only two powers relating to foreign affairs: he is commander-in-chief of the armed forces, and he has the power to receive ambassadors.
