笛卡兒將實體區分成物質以及非物質的心靈兩大類。然而，姑且不論心靈究竟是否屬於非物質，我們一樣可以追問，何謂「非物質實體」？以及真有這樣的東西存在嗎？為了不要讓「非物質」這個概念等同於「非存在」，我們必須將「物質」做一個比較明確的界定，以及當我們發現什麼東西的時候，可以判定非物質實體的存在。本文主張，只要我們可以找出一種完全不屬於當今物質界定下的東西，就可以暫且稱呼它們為非物質實體。依據這樣的界定，以及量子力學中的雙狹縫實驗結果，我認為，我們目前有不錯的理由可以宣稱非物質實體的確存在。而且，此非物質實體明顯和心靈意識息息相關，可以作為我們所尋找的「靈魂」的候選員。

Descartes divided substances into two types – the material objects and the immaterial mind. However, even if the mind is not really immaterial, we can still ask whether immaterial substance exists and what the immaterial substance is? In order to avoid the situation that the concept “immaterial” equals to “non-existent”, we have to give it a clear definition. In what situations can we declare that the immaterial substance exists? In this paper, it is suggested that once something that does not belong to and highly differ from the material we know today can be found, we can call it “immaterial substance”. According to this definition and the double-slit experiment in quantum mechanics, it is proposed that we have good reasons to declare immaterial substance exists. In addition, this immaterial substance is clearly related to the mind, and it could be the “soul” that we are looking for.