The philosopher, Descartes is famous for his dictum, cogito ergo sum, I think, therefore I am. Inasmuch as there is a phenomenon of hallucination, in which a person sees or hears things that do not exist, yet one is absolutely certain of their existence, Descartes raises the possibility, remote as it may be, that perhaps I am hallucinating. Nothing in the world exists, nothing that I see, hear, touch, taste or smell. I may be hallucinating all these things. My body may be a hallucination. Perhaps I don't have arms or legs, and when I pinch myself to make sure I'm awake, that too may be hallucinatory. However, one thing is undeniable. Even if I am hallucinating everything, I must exist in order to be hallucinating. Perhaps I don't exist as a body, because that may be a hallucination. However, in some shape or form, I must exist, otherwise I could not be hallucinating. Hence, cogito ergo sum, I think, therefore I am.
Alongside hallucination as an error of reality testing stands "delusion." Delusion is a fixed belief from which a person cannot be swerved by logical argument. A man who had the delusion that he was dead was brought to a psychiatrist. The psychiatrist asked him, "Do dead people bleed?" The man said, "Of course not." The psychiatrist then had him recite 100 times, "Dead people to not bleed." He the pricked the man's finger, causing it to bleed. The man said, "Dead people do bleed."
Rebbe Yeruchem Levovitz of Yeshivas Mir (Daas Chochma Umussar vol.2 pp. 139-142) cites the Talmud which states that Iyov claimed that "The world was turned over to Satan" (Iyov 9:24), and states that this indeed is so. Satan was given the extraordinary power to delude people and even to cause them to hallucinate.
When Moses did not return from Sinai at the expected time, Satan told them that without food and water, Moses had perished, and caused them to have a vision of Moses being carried on a bier (Shabbos 89a). With their own eyes, they saw that Moses had died!
The Torah says that there may arise a false prophet who will claim that G-d has commanded us to worship an idol, and to prove his authenticity the prophet will perform miracles. The Torah cautions us to beware of this, because this is Hashem's testing our loyalty to Him. Rebbe Yeruchem states that the ability to perform miracles to justify idolatry is an example of the power to delude that was given to Satan.
Rebbe Yeruchem says that we live in a world of delusion. Moses warned us against thinking that our success is due to our own prowess. "You may say in your heart, 'My strength and the might of my hand made me all this wealth'" (Devarim 8:17). Yet, we think that if we spend ten hours a day at work, we will earn more than if we spent four hours a day at work, as if the time we invest in work determines how much we will earn. Everyone thinks this way, but it is a delusion. Satan has caused us to think that our success depends on our effort.
In Yellowstone National Park there is a geyser, Old Faithful, that erupts regularly. Two pranksters brought a steering wheel and shaft, and placed themselves where they could be seen by tourists. The guide was telling tourists about the eruptions of Old Faithful, and just as the geyser was due to erupt, one prankster shouted to the other, "OK! Let ‘er go!". The other prankster gave the wheel a sharp twist, and then the geyser erupted. Observers could believe that the eruption was fabricated mechanically.
So it is with us. Torah teaches us that, with the exception of choosing good or evil, which Hashem has left to the individual, Hashem controls everything, from the minutest to the most cataclysmic events. However, we do not live our lives according to this belief. Rather, we believe that there is much that we can control. This, Rebbe Yeruchem says, is the work of Satan, who causes us to be deluded.
So, Descartes tells us that we may all be hallucinating, and Rebbe Yeruchem says that we all live one massive delusion. Whereas we need not live our lives according to Descartes' philosophy, Rebbe Yeruchem says that we must free ourselves of Satan's delusion. The only true reality is what Torah says is true. Everything else is delusional.
Ramchal in Mesilas Yesharim says that we were placed in a world replete with nisyonos (trials and tests). Rebbe Yeruchem says that accepting what Torah says as the true reality and resisting the Satanic delusion is the nisayon with which we are all tested.