"... At first blush Jesus appears to be making a physiological claim about the role of the eye, but the saying is sandwiched between two sayings that set pursuit of money and pursuit of God in opposition to each other. Moreover, the word ἁπθμῦξ was rarely used in physiological descriptions, but it was frequently used in an ethical sense. Thus it would appear that, as in 5:25-26, the humdrum imagery of the saying conceals a deeper theological claim. Much has been written about this saying and its Lukan parallel (11:34-36). There are two key exegetical questions. The first is the meaning of ὁ θύπκμξ ημῦ ζώιαηόξ ἐζηζκ ὁ ὀθεαθιόξ. Allison notes that modern exegetes have incorrectly assumed that this refers to the eye acting as a window that allows light into the body. Ancient Jews, however, thought that the eye contained its own light, hence the metaphor of a lamp, which is a source, not a channel, of light. (Examples include 2 Samuel 12:11; Sirach 23:19; Daniel 10:6; Testament of Job 18:3) Allison goes on to read the conditionals in 6:22b-23 so that the apodosis entails the realization of the protasis, rather than the reverse as is usually the case ..."

Eubank, Nathan Paul Wages of Righteousness: The Economy of Heaven in the Gospel According to Matthew (pp. 90-91) Duke University, 2012