Put simply, only by "pushing the envelope" can you find out what you are capable of. Eliot here is saying that only when humans test their potential by "going too far" can they discover their truest abilities and possibilities. In essence, this quote echoes the "carpe diem" (seize the day) sentiment found in other great idioms and expressions from literature. Risk is inherent to gain, to put this another way. "Nothing ventured, nothing gained," another saying says, and Eliot's quote says the same thing more eloquently. Like other great poets, Eliot is encouraging readers to test limits and boundaries.