Perhaps at some time, you have considered like this, "How do I know anything outside of myself actually exists?"
Well, you wouldn't be the first person to ever ponder like that. There is a whole school of transcendentalists that think like that. And have a very elaborate philosophy constructed around it. Teaching during the eighth-century in India, Shankaracharya's influence was (and still is to today) very widespread.
In reading the writings of Baladeva Vidyabusana whose teachings based on the Vedas contradict this idea I came across this verse where he first quotes Sankaracharya's premise and then defeats it.
Here is the quote:
4.4a The impersonalist followers of Sankaracarya proclaim:
"I, the living entity, am the only supreme, and other living entities, as well as the Supreme Personality of Godhead, do not actually exist but are only
imagined in the mind when there is ignorance of my actual nature."
And here is the counter-argument:
4.4b The Katha Upanisad (2.2.13 ) explains: "Of all eternals, there is one who is the chief eternal. Of all conscious living entities, there is one who is the chief conscious entity. That supreme living entity, the Personality of Godhead, maintains the others, and fulfills their desires according to their merits. Only saintly persons, who can see, within and without, the same Supreme Lord, can actually attain to perfect and eternal peace."
Prameya Ratnavali, Fourth Prameya 4a and 4b
http://www.krishnapath.org/Library/Goswami-books/Baladeva-Vidyabhusana/Baladeva_Vidyabhusana_Prameya_Ratnavali.pdf