{Single} (haplous). Used of a marriage contract when the
husband is to repay the dowry "pure and simple" (	(886e) phern(886e)
hapl(886e)), if she is set free; but in case he does not do so
promptly, he is to add interest also (Moulton and Milligan's
_Vocabulary_, etc.). There are various other instances of such
usage. Here and in  Lu 11:34  the eye is called "single" in a
moral sense. The word means "without folds" like a piece of cloth
unfolded, _simplex_ in Latin. Bruce considers this parable of the
eye difficult. "The figure and the ethical meaning seem to be
mixed up, moral attributes ascribed to the physical eye which
with them still gives light to the body. This confusion may be
due to the fact that the eye, besides being the organ of vision,
is the seat of expression, revealing inward dispositions." The
"evil" eye (pon(8872)os) may be diseased and is used of stinginess
in the LXX and so haplous may refer to liberality as Hatch
argues (_Essays in Biblical Greek_, p. 80). The passage may be
elliptical with something to be supplied. If our eyes are healthy
we see clearly and with a single focus (without astigmatism). If
the eyes are diseased (bad, evil), they may even be cross-eyed or
cock-eyed. We see double and confuse our vision. We keep one eye
on the hoarded treasures of earth and roll the other proudly up
to heaven. Seeing double is double-mindedness as is shown in
verse  24 .