Sense of Pain

John Watson really was an excellent doctor.

John wasn’t Sherlock-level observant by any means, but one day they were sitting in a cafe drinking coffee and John started discreetly pointing at people and diagnosing them. Carpal tunnel, back pain, hip replacement, scoliosis. It’s not that Sherlock forgets that John’s a doctor as well as a soldier. It’s just that sometimes he doesn’t remember until John reminded him.

John frequently patched up Sherlock’s various scrapes and bruises that he collects on cases. He’s gentle but firm, good with a needle when Sherlock needs stitches. He made Sherlock eat and sleep, which Sherlock rebelled against, but it’s really more for the sake of rebelling than of any real desire to thwart John’s ministrations.

Sense of Temperature

John Watson made the flat seem warmer, just by being in a room.

Sherlock knew that the temperature change from one person being in a room would be negligible to the point of immeasurability, so he knows that John doesn’t literally make a room warmer by being in it. It just seems that way.

And then it was Sherlock’s turn to fall in the river. When John helped to pull him from the water, Sherlock’s lips were going blue and he was shaking with cold. John pulled off Sherlock’s sodden jacket (he’d shed the coat before he’d gone in the water) and wrapped one blanket around his shoulders and another around his legs. He curled up as much as he could, trying to warm himself as John dried his hair with yet another blanket. But the shaking wouldn’t stop, so John pulled off his own coat and unwrapped the top blanket and wrapped his arms around Sherlock before tugging the blanket around both of them. John breathed against Sherlock’s neck, the warm air sending goose bumps down Sherlock’s spine. The resulting shiver wasn’t due completely to the cold.

Sense of Balance

John Watson liked to keep things on an even keel.

Sherlock appreciated the stabilizing influence John has on him, even if he didn’t always show it. Didn’t ever show it, truth be told.

John was tea when he’s agitated and coffee in the mornings and blankets when he’s cold and food at least once a day, usually more. John was sleep and caretaking and responsibility. He’s crap telly on rainy days and good Chinese after a case. He’s medicine and bandages and steady hands and the comforting sensation of having another person looking out for you.

And, best of all, he was a conscience.

Proprioception

Sherlock saw John Watson in Moriarty’s hands, and it was as though he’d lost one of his own limbs.

If Moriarty was to kill John, he wouldn’t just be burning Sherlock’s heart. He’d be amputating a hand, an arm, a balancing force, the warmth of companionship, Sherlock’s shield against the world.

Sherlock always knew where John was afterwards. Periodically, when he realised John wasn’t in the same room, he would stop and listen, only resuming his pacing or experiment when he heard the creak of floorboards or a cough or a snore. Once, just days after Moriarty and the pool, Sherlock had woken to find John gone from the flat and John’s phone on the coffee table. He’d paced for half an hour until John returned carrying the shopping. John stopped in the doorway, startled to see Sherlock, and Sherlock breathed a sigh of relief. John’s mouth twitched in a humourless smile and he’d apologised for leaving his phone.

Mycroft sneered at caring, at sentiment. Sherlock knew that it wasn’t an advantage, it was a weakness, but once the caring started it was so hard to stop. 

~~~

The titles of these two parts I borrowed from something Hank Green said in a video on YouTube. The video can be found here, the part I'm talking about starts at around the two minute mark: http://www.youtube.com/watch?v=Yj0eXvMpnak

There are other senses that the human body has (sense of hunger, etc.) but I chose these because they're kind of internal senses that can be influenced by external stimuli.