Plato's aetiological search in the Phaedo (95e7-102a9) has been recently interpreted as a conceptual reformation of the term 'aitia'. Accordingly, many scholars attempt to look into the presuppositions of the Platonic aitia, in particular his three tacit principles of aitia, in order that they may adduce evidences of a semantic modification. In this paper, I criticize the above interpretation as misleading and call attention to Plato's original criticism of the physical aitiai (96e5-97a5). Plato there avows his lack of understanding that 'where one is added to one either the one to which it is added or the one that is added becomes two, or that the one added and the one to which it is added becomes two because of the addition of the one to the other', and wonders (thaumazo) that 'when each of them is separate from the other, each of them is one, nor are they then two, but that, when they come near to one another, this is the cause of their becoming two, the coming together and being placed closer to one another' (tr. by Grube). His perplexity here strikes us as odd, but that the physical conjunction stated in the passage is a typical formulation of change for natural philosophers provides a good account for it. This is because by the disavowal of understanding the phenomenon, he can be considered as indicating the inexplicability of change on the natural philosophers' side. For this reading, I offer the following reason: having no explicit account of the ontological status of properties such as 'two' or 'beautiful', natural philosophers are compelled to admit that any property belongs to some physical thing by itself, so that there is no room for 'property change' in their world, as Aristotle also points out in G.C. I. 314b15-28. This is because if the thing that is intrinsically one becomes two, it would be one and two by itself, which sounds absurd immediately. Similarly, every change to opposites would be impossible on that physical conjunction model (cf. perdurantist's attack against endurantist in contemporary metaphysics). Plato thus introduced Forms, such as 'the beauty itself, and replaced physical conjunction by formal participation with a view to providing a coherent account of change. By doing so, he makes an ontological claim that Forms and participations can solely bring about change. It is therefore this ontology-centred argument that led Plato to the formal aetiology, whereas a conceptual concern is to be considered marginal. In other words, the alleged conceptual reformation of 'aitia' was in fact supervened upon the onto-metaphysical reflexion on change, stimulated by his ingenious 'wonder'.