Many environmentalists have urged environmental awareness on consumers, saying that if we accept moral responsibility for our effects on the environment, then products that directly or indirectly harm the environment ought to be avoided  Unfortunately it is usually impossible for consumers to assess the environmental impact of a product, and thus impossible for them to consciously restrict their purchases to environmentally benign products
Because of this impossibility there can be no moral duty to choose products in the way these environmentalists urge, since
a person cannot possibly have a moral duty to do what he or she is unable to do