Of all the deep and dark strategic and psychological angles to be explored as the Celtics head into Game 6 against the Knicks tonight, there is a rather basic human reflex that unfolded among three old guys with not too many gunfights left. When the future is uncertain, one tends to cling to the present. Put simply, Paul Pierce, Kevin Garnett and Jason Terry don’t want to stop playing basketball with each other just yet, dammit. At ages 35, 36 and 35, respectively, and with the Celts headed for some major offseason decisions that could very well alter the franchise’s facade next season, the three have taken a look around and decided that a quick exit to the Knicks — these Knicks — was not how they wanted their potential final paragraph together to read. “More important than all of this stuff people are talking about is how do we want to go out?” said Terry. “If we’re going to go out, we’re going to go out fighting and with our whole entire effort out there on the floor. I think that’s been more important than anything in getting us from where we were to here.” While Pierce and Garnett have been together since the serendipitous Celtics summer of 2007, Terry is a newcomer to the North End Home for Aging Basketeers. As such, he wasn’t quite used to the history of getting into postseason trouble. (Or do you forget that the great and powerful 2008 champions were taken to seven games by the Hawks?)