The Bible says "As a man thinketh in his heart, so is he." For example, an avaricious man may appear generous, but he is avaricious, regardless of appearances. In short, men do not change.
On the other hand, Allen tells us positve thinking changes us for the better. If we think about worthwhile things, have worthwhile goals, we will become better men.
Same sentence, different emphasis.
As A Man Thinketh is an earlier version of The Power of Positive Thinking and the more recent best seller, The Secret.