Why won't Brutus take an oath?

Brutus says,

"No, not an oath. If not the face of men,

The sufferance of our souls, the time's abuse--

if these be motives weak, break of betimes,

And every man hence to his idle bed."

Brutus believes that each man should be trusted based upon their integrity, and who they are. He means that is they need to take and oath in order to deem their honesty and intent...... they shouldn't be doing what they plan to do, as there can be no question thast they are each invested in carrying out Caesar's execution.