Weak sustainability acknowledges that some forms of natural capital can be substituted by human-made capital, as long as the overall capital stock remains constant or increases. Strong sustainability maintains that certain forms of natural capital are irreplaceable and should not be traded off for human-made capital. It emphasizes the intrinsic value of nature and the need to protect essential ecological functions. "If a forest is depleted for timber, sustainability suggests that planting a tree farm or investing in technology for sustainable logging could be an acceptable substitute to maintain overall capital." weak strong