It belongs to the nature of man, as from Heaven, to be still at his birth. His activity shows itself as he is acted on by external things, and developes the desires incident to his nature. Things come to him more and more, and his knowledge is increased. Then arise the manifestations of liking and disliking. When these are not regulated by anything within, and growing knowledge leads more astray without, he cannot come back to himself, and his Heavenly principle is extinguished.
人 = rén = person, human, human being
生 = shēng = birth, born, alive, exist
而 = ér = and, then, implies sequential/causal relationship
靜 = jìng = quiet, still, motionless
天 = tiān = heaven
之 = possessive
性 = xìng = (innate) nature
也 = yě = also
Paraphrase: Humans are born with a peaceful, qentle nature. The nature of Heaven is also like this.