At the end of Parshas Shlach (15:39), the Torah cautions, "Do not explore after your heart and your eyes, after which you stray". Chazal (Berachos 12b) explain that "after your heart" refers to heretical beliefs. At first glance, heresy is a product of the mind, not the heart. Why do Chazal associate inappropriate thoughts with the heart?
Rav Elchonon Wasserman (Dugma'os L'Biurei Aggados Al Derech Hapshat, printed at the end of Koveitz He'aros, chapter 1) answers that, in reality, belief in G-d should be something that comes naturally. After all, the incredible precision, sense of order, and exquisite detail found in every living thing - in plants and animals and especially in human beings - conclusively point to the existence of a Creator. Those who choose not to believe in a Creator do so only because their hearts refuse to accept that which is self-evident. The Torah advises us to follow our minds, not our hearts, to ensure that we view the world from the proper perspective.
Often, people believe what they want to believe; they see what they want to see. They filter their experiences through the prism of their deep-seated emotions and fears, their likes and dislikes. This was the mistake of the spies. The posuk (13:26) says, "They went and they came". Rashi comments that the Torah compares their going to their coming; just as they came back with an evil scheme, so too they went initially with bad intentions. For whatever reason, the spies did not want to enter Eretz Yisrael, and that colored their whole perspective. They saw a land with strange, gigantic fruit (see Rashi 13:23), a land "that eats its inhabitants" (13:22). They could not see the beauty of the land nor Hashem's beneficence (see Rashi 13:22) because their hearts did not want to see the positive.
Similarly, Korach's jealousy of Aharon caused him to make an error in judgement. Moshe Rabbeinu warned Korach that only one person will remain alive after offering incense (Korach 15:7). Korach was a smart man, so why did he insist on taking such a risky gamble? Rashi explains that his eye deceived him. He saw an impressive line of descendants, including the prophet Shmuel and twenty-four watches of kohanim, emerging from him. But he did not see that his sons would do teshuva and this chain of greatness would emerge from them, not from him. Korach interpreted his vision incorrectly because his jealous heart refused to allow him to see things differently.
A mistake of the heart is also what led to Haman's downfall. When Achashveirosh asked him what should be done for the man whom the king wishes to honor, Haman "said in his heart" whom would the king wish to honor more than me? (Esther 6:6) Why was Haman so convinced that there was no one else the king would want to honor besides him? And why did he suggest such an outlandish idea - to dress the man in the king's clothes and ride him on the king's horse? Isn't that a little extreme even for someone the king wishes to honor? The answer is that Haman was so arrogant he just could not control himself. His desire for honor clouded his vision. And that was the beginning of his downfall because once Achashveirosh saw that Haman had delusions of grandeur, he was only too happy to get rid of him when Esther told him of Haman's plot.
People often hear what they want to hear; they see what they want to see. If one allows his feelings of jealousy or his desire for pleasure and honor to color his perceptions, that can lead him to a bad place. But if one listens to the inner voice that emanates from his neshama, then he will be motivated by a higher sense of purpose, by positive middos and healthy emotions, and that can lead him to great heights.
The mitzvah of tzitzis serves as a constant reminder to keep the proper perspective. "And you shall see them and remember all the mitzvos of Hashem" (15:39). Seeing the tzitzis constantly encourages us to improve our character and purify our vision as much as we can so that we will view the world through the right lens. We will appreciate the opportunities we have to accomplish and to grow spiritually; we will develop a proper sense of priorities; and we will make our life decisions guided by a Torah outlook.
Two people can see the same thing. But how they interpret what they see is often a matter of perspective.