Hmmm... As someone who is a materialist, I guess I'd put the concept of higher and lower self in terms of perspective.
Lower self would be the basic, instinctual, subconscious, and first hand perspective thoughts we have and our immediate concerns. It considers the subjective world we interact with directly.
Higher self would be thoughts that consider things outside of the self and remove the self from the equation. It considers others, it considers greater consequences, and it considers the inner workings for how things function in reality. It considers the objective world we can't interact with directly.
To me, lower doesn't mean lesser, and higher doesn't mean greater. They both have their place and time to be utilized. They are merely a shift in thought from ego centric to ego suspended.