No one believes in T-E-A-M more than B-I-L-L Belichick. But even he apparently knows it has its limits. Midway through delivering the keynote address yesterday at the opening of a three-day sports medicine seminar at the Sheraton Boston co-chaired by Patriots head team physician Dr. Tom Gill and Eagles team physician Dr. Gary Dorshimer, Belichick was explaining his theories on “Building a Champion’’ when he looked at an old cliche from a new perspective. “People say all the time there’s no ‘I’ in team,’’ Belichick said. “That’s true. But there’s an ‘I’ in win. Within a team, strong individual performance determines whether or not we win. You can stand around all day and hold hands in the locker room chanting ‘Team, Team, Team.’ That’s not going to do anything. “Individual performance is critical to winning. Individual toughness is critical to winning. Every member of our team is a shareholder. Are they all exactly equal? Of course not.’’ Yet even the critical nature of individual performance in team success is, in Belichick’s mind, enfolded inside a larger concept he defined as “mental toughness.’’ That is an oft-used phrase in Foxboro, especially by Belichick, first lieutenant Tom Brady and master sergeant Vince Wilfork. It is the same phrase predecessors holding those middle management positions like Rodney Harrison, Willie McGinest and Tedy Bruschi during the Patriots’ Super Bowl years, often referred to. But what, exactly is “mental toughness’’ in the world of Bill Belichick? Does it mean you are a feared tough guy? Does it mean you play through levels of pain normally tolerated only by Spartacus? Does it involve delivering flamboyant pregame speeches to inspire like the Ravens’ Great Orator Ray Lewis? Is mental toughness simply someone who keeps his head when all those around him are losing their minds? To a degree perhaps these are all part of the whole but in its most finite terms Belichick described “mental toughness’’ as one very difficult concept. “Mental toughness is doing the right thing for the team when it’s not the best thing for you,’’ Belichick said. “That’s what defines mental toughness in my mind.’’