In Aaron B. Cooley v. The Board of Wardens of the Port of Philadelphia, plaintiff in error Aaron B. Cooley argued that a state law requiring pilots to pay fees for their licenses was unconstitutional as it violated the Commerce Clause and impeded interstate commerce by taxing goods from other states entering Pennsylvania ports. The Supreme Court disagreed with this argument, ruling that while Congress had exclusive power over foreign and interstate commerce, states were allowed to regulate local matters such as pilotage within their own borders so long as they did not interfere with federal laws or regulations concerning navigation and trade between different states or countries. This decision established an important precedent regarding the scope of state powers when regulating activities related to interstate commerce.

In the dissenting opinion of Aaron B. Cooley v. The Board of Wardens of the Port of Philadelphia, Justice McLean argued that Congress had no authority to pass a law which would grant exclusive privileges and immunities to certain individuals or groups in violation of the Constitution's Commerce Clause. He believed that such laws were unconstitutional because they interfered with interstate commerce by allowing one state to impose its own regulations on vessels entering from another state, thereby creating an undue burden on trade between states. Furthermore, he argued that if Congress was allowed to pass such laws then it could potentially create monopolies and other forms of economic discrimination against citizens from other states who wished to engage in business activities within their borders. In conclusion, Justice McLean maintained his belief that any law passed by Congress must be consistent with the principles laid out in Article I Section 8 clause 3 (the Commerce Clause) and should not interfere with free trade among all US States for fear it will lead down a slippery slope towards further economic inequality amongst citizens across different regions.