What did John Byro mean when he said, A suspicious man would believe his eyes instead of his heart.

A suspicious man would tend to believe the evidence that is present in front of him and in John Byro's case,he could have accused Aram and Maroud of stealing his horse because the one which they had was exactly like his stolen horse. But Byro was not a suspicious man,he was aware of the fact that the boys' family was famed for their honesty, it had been the watermark of their family for several generations. So he takes the boys at face value and chooses to believe in what he knows by heart and not what he sees in front of him.