brainless

Having no brain.

Surely it was more likely that a brainless creature, acting solely by vague predatory instinct, would give up the chase when I disappeared, and, after a pause of astonishment, would wander away in search of some other prey?

Unintelligent, with little common sense.

"I don't mean all of your friends—only a small proportion—which, however, connects your circle with that deadly, idle, brainless bunch—the insolent chatterers at the opera, the gorged dowagers, the worn-out, passionless men, the enervated matrons of the summer capital,[…]!"

Overly repetitive and unchallenging, not requiring any careful attention or providing any significant stimulation.

Near-synonyms: automatic, rote, dull