The wealth effect, interest-rate effect, and exchange-rate effect all help explain why the aggregate demand curve is downward sloping. Phyllis, a fellow student in your AP Macro class, suggests that the substitution effect that you learned about in Unit 1 also explains why aggregate demand curve is downward sloping. She reasons that since it explains why a market demand curve is downward sloping, it must also be valid for aggregate demand. Explain why Phyllis is wrong.