In the context of the show, I'd say it's suggested that he does. When Ira Graves possesses him, or the entity from Power Play or even the whatever it was in Masks, Data's personality is suppressed just as if he were any other individual in a similar situation, like O'Brien was, or Troi on a few occasions. What is that which is gone if not the spirit within him?
As far as really
defining it.... I wouldn't even know where to begin. I can look at the dog & see something of a soul at work there........ I think