Why should we believe that we are one without a second?

In a Zoom meeting with the Ramana Maharshi Foundation UK on 27th January 2024, Michael answers various questions about Bhagavan’s teachings, and the first question he answered was: "When describing Bhagavan's teachings, Michael quotes "ekamevadvitiyam" from Chandogya upanishad. But, similar to the lucid description of the rest of the teachings, is there a methodical logical statement for advaitam? In other words, why should we believe the reality is one without the second?" This episode can also be watched as a video here and a more compressed audio copy in Opus format (which can be listened to in the VLC media player and some other apps) can be downloaded from here.