A frequent fantasy in American politics — now brought to reality by the ascension of Donald Trump — is that a person without real subject matter knowledge could be an effective president by relying on skilled subordinates. Trump made a version of this argument on the campaign trail, arguing that he would “hire the best people” for key roles in his administration. And lots of Americans have taken solace in the fact that many of Trump’s key Cabinet appointees do, indeed, appear to be knowledgeable in their issue areas.

A weekend Trump tweet about North Korea, however, revealed exactly why this won’t work. Policy topics feature complicated linkages across subject areas that lead to inherent conflicts in the perspectives of different government agencies.

Why would I call China a currency manipulator when they are working with us on the North Korean problem? We will see what happens! — Donald J. Trump (@realDonaldTrump) April 16, 2017

What Trump is saying here, which is reasonable enough, is that he’s chosen to prioritize nonproliferation concerns that require cooperation with China over an economic policy agenda that would have required confrontation. That’s more or less how the Obama administration saw it too. And even though candidate Trump promised the opposite, it’s probably good that he’s come around. The downsides of nuclear war are much worse than whatever gains we’d get from playing hardball with China on currency.

But it’s not just North Korea where two issues are in conflict. All kinds of policy issues have this feature of requiring a balance of competing considerations.

Governing is about balancing priorities

On immigration enforcement, for example, a crackdown on “sanctuary city” policies may be optimal for law enforcement professionals, since it could increase cooperation between local authorities and the immigration officials and lead to more apprehensions. But Trump’s approach is already having a deleterious impact on public health by discouraging mixed-status families from seeking health care for people who are eligible. And a traditional concern of law enforcement officials has been that involving police departments in immigration enforcement will make it harder to secure immigrants’ cooperation with criminal investigations.

And in trade policy generally, protectionist approaches could help Americans who work in sectors that compete heavily with foreign imports (largely the manufacturing of consumer goods) while hurting Americans who work in sectors that compete heavily with foreign producers for global market share (agriculture, for example).

This is why, at the end of the day, there’s no alternative to the president of the United States actually doing the day-to-day work of the job. It’s fundamentally unlike serving as the CEO of a business conglomerate, where you can simply pick one trusted subordinate to run a golf club and another trusted subordinate to manage a hotel and then fire them if they don’t bring in the kind of profits you expect. Different kinds of policy goals conflict with one another, and need to be integrated by the White House staff and ultimately by the president himself.

In an ideal world, that would take the form of a knowledgeable chief executive who’s disciplined enough to accept briefings and listen to a wide range of opinion before reaching decisions. Instead, we have Trump.