S1: True knowledge is true power. S2: A little knowledge however is a dangerous thing. The second sentence:

The first sentence praises true knowledge as power. The second sentence doesn't deny this but adds an important condition - that incomplete or shallow knowledge is dangerous. The word 'however' signals a qualification - the assertion holds only when knowledge is true/complete. This limits and refines the first statement without contradicting it. Therefore, the second sentence qualifies the assertion of the first.