Jack remembers through taste.
Coffee in the morning when things were simple and they could forget that the world outside of their little bubble existed.
Salty sweat on his tongue when Ianto was too lost in Jack to care about his unusually ruffled appearance or the wanton sounds spilling from his mouth.
Scotch and dust after a long day when they were both too dirt splattered and exhausted to do anything but kiss with hunger and chapped lips.
And when Jack realises that he's now using past tense and that one day he won't remember at all, it breaks him.