嬰兒和成人在外表上其實具有很大的差異，毛毛蟲和蝴蝶也是如此，但這不妨礙我們將這些東西歸成一類，形成概念。雖然形態改變，但是仍維持在同一個向度中，這個是事物的「本質」(essence)。人們因為直覺(naive intuition)而形成特定類別，叫作心理本質論(naive essentialism或psychological essentialism)。這並不是說它完全知道本質為何。例如知道「水」這個概念，並不需要知道它是組成分子為何，只要相信它存在某些特質即可。相信某個概念存在某個特質就是科學探索的起始。
相信某個類別存在某些特質以及宣稱存在某某特質是不一樣的。
和現代生物的發展來看，本質論可能會有一點點奇怪，畢竟亞里斯多得那個時候認為每一個物種的本質都是固定而沒有變化的。不過，本質論也使我們去思考為什麼某些事物會歸到同一個類別：「鳥」不只是因為牠們的外觀相似，牠們在更深層的地方是有許多特質是相同的。在不同文化下的科學和民間理論對於事物的本質存在不同的理解，這是把本質論放到現代生物之所以奇怪的地方(Atran, 1998) 。
從洛克的角度來看，本質論者相信一個已經存在的類別，類別下的事物在外表的特質下仍潛著未見的特質。這可以在跨文化的研究或小孩子的身上發現。小孩不會因為三角龍的外形而認為牠和犀牛同屬於溫血動物，反而因為其和雷龍有同樣是「恐龍」的特質，而認為是冷血動物(Gelman & Markman, 1986; Gelman & Markman, 1987) 。
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