At the end of the tochacha, Hashem guarantees us, "I will remember My covenant with Yaakov and also My covenant with Yitzchak, and also my covenant with Avrahom will I remember" (Vayikra 26:42). Rashi points out that regarding the covenants of Avrahom and Yaakov Hashem states that He will remember them, while the word "remember" is not mentioned in conjunction with the covenant of Yitzchak.
Rashi cites Chazal who explain that one only needs to use their memory to remember something which he does not presently see in front of him. Accordingly, Hashem does not need to recall and remember the covenant He created with Yitzchak, since He sees Yitzchak's ashes [from the Akeidah] piled up on themizbeiach in front of Him.
What does this mean, asks Rav Wolbe (Shiurei Chumash Parshas Vayeira 22:14)? We all know that Yitzchak was not actually sacrificed and burnt on themizbeiach, and obviously no ashes were created. He answers that the Navi states, "And a book of remembrance was written before Him for those who fear Hashem and who give thought to His Name" (Malachi3:16). Chazal ask (Kiddushin 40a) to whom is the pasuk referring when it mentions those who give thought to Hashem's Name? They answer that it refers to those who had a true desire to perform a mitzvah but circumstances that were out of their control prevented them from actually fulfilling their intention. Hashem considers their desire as if the mitzvah was performed, and thus the mitzvah is written down in the book of remembrance before Him.
Indeed, Yitzchak was not actually burnt on the altar. Nevertheless, the intense and true desire that he had to perform the mitzvah was accredited to his account, exactly as if it had come to fruition. The most essential aspect of the mitzvah is the desire and therefore even if one is prevented from doing the mitzvah Hashem considers it as if the mitzvah took place. Rav Wolbe adds that understandably one who has a thought to do a mitzvah and does not perform it despite the lack of outside interference, does not fall into the above category. Had he truly had the desire to perform Hashem's will, he would have followed through and would have done it.
This idea is an eye opener regarding the proper way to approach a mitzvah. It seems quite possible that a person who did not actually perform a mitzvah will receive more reward than his counterpart who actually performed that mitzvah but without a true desire! Our desire makes all the difference. Accordingly, one who truly wishes to spend more time learning, davening or performing chessed, but is precluded from doing so because of business or familial obligations, will merit books full of mitzvos to be accredited to his name in the World to Come!