Some of the definitions of these parameters in the Drake equation look a bit biased towards anthropocentrism (or perpahs earthlingcentrism) because... what does "habitable zone" mean? Habitable by whom? We don't know the many kinds of life that might exist in the universe. Perhaps the actual parameter should be "Planets (or asteroids, etc) where life appears" and "Planets with multicellular life". I think that intelligence depends on complex life with tissues and organs and once life surpass certain level of complexity it's very likely that some level of intelligence will appear. Then language (as symbolic language like we humans have) would be the next parameter. What do you think? Does it makes sense to you, guys?