A.T. Robertson Commentary Luke 20:5

"And they reasoned with themselves, saying, If we shall say, From heaven; he will say, Why did ye not believe him?" — Luke 20:5 (ASV)

They reasoned with themselves (συνελογισαντο). First aorist middle of συλλογιζομα, to bring together accounts, an old word, only here in the N.T. Mark and Matthew have διελογιζοντο (imperfect middle of διαλογιζομα, a kindred verb, to reckon between one another, confer). This form (διελογιζοντο) in verse 14 below.

If we shall say (εαν ειπωμεν). Third-class condition with second aorist active subjunctive. Suppose we say! So in verse 6.