Either make ( ἢ ποιήσατε). Not "suppose" (fac, pone), still less "declare," but "make." The Lord is speaking in a parable. You would not, surely, make a tree in any other way; it would be against nature; how then imagine it can be so in your own persons?
Matthew 7:18 and Luke 6:43 state as a fact that the reverse case does not take place in nature. The tree good, and his fruit good (i.e. one if the other); or else make the tree corrupt (Matthew 7:17, note), and his fruit corrupt: for the tree is known by his fruit.
"By his own fruit" (Luke).