Concentration
Samādhi Sutta  (SN 35:99)

“Develop concentration, monks. A concentrated monk discerns things as they have come to be. And what does he discern as it has come to be?

“He discerns, as it has come to be, that ‘The eye is inconstant’ … ‘Forms are inconstant‘ … ‘Eye-consciousness is inconstant’ … ‘Eye-contact is inconstant‘ … ‘Whatever arises in dependence on eye-contact—experienced either as pleasure, as pain, or as neither-pleasure-nor-pain—that too is inconstant.’

“He discerns, as it has come to be, that ‘The ear is inconstant’ … ‘The nose is inconstant‘ … ‘The tongue is inconstant‘ … ‘The body is inconstant’ …

“He discerns, as it has come to be, that ‘The intellect is inconstant’ … ‘Ideas are inconstant’ … ‘Intellect-consciousness is inconstant’ … ‘Intellect-contact is inconstant’ … ‘Whatever arises in dependence on intellect-contact—experienced either as pleasure, as pain, or as neither-pleasure-nor-pain—that too is inconstant.’

“So develop concentration, monks. A concentrated monk discerns things as they have come to be.”

See also: MN 52; SN 22:5; AN 3:74; AN 4:41; AN 5:28; AN 9:36