Lyric discussion by showyourscars 

Cover art for Arabella lyrics by Arctic Monkeys

Actually, he sings:

"Just might've tapped into your mind and soul, You CAN be sure".

Just listen closely.

If you take the emotional vibe and the lyrics all together, and Alex's certainty about who Arabella is to him, and how bad he wants her, you can tell he digs her, definitelly - so he sings that way, not the other.

My Opinion

*definetely

It's "can't" according to the lyrics on the album :)

Even if it was "can be sure" that line is sung in a way that suggests he's "sure" in the way anyone who is intoxicated can be sure, until they sober up. Either way, it sounds certainly uncertain.