Complex sentence: He posited that the mind is a "blank slate" or "tabula rasa", free from innate ideas, and that because children enter the world without preconceived notions, whatever ideas they absorb early in life will fundamentally affect their later development.
Split into simpler sentences: He posited that the mind is a "blank slate" or "tabula rasa", that is, it is free from innate ideas. Thus, because children enter the world without any preconceived notions, whatever ideas they absorb early in life will be fundamental to their later development.