Halliday & Matthiessen (1999: 489):
In other words, a figure of sensing, configured as Senser + Process + Phenomenon, can be represented either as the Phenomenon impinging on the Senser’s mental processing (effective: Phenomenon as Agent) or as the Senser’s mental processing ranging over the Phenomenon (middle: Phenomenon as Range). This kind of a reversal of perspective on the figure (like/please, fear/frighten, believe/convince, etc.) is a special feature of sensing and one important reason for recognising it as a distinct type. (This is also true of the metaphorical variant; cf. John was afraid of the dog : The dog was scary [to John].)