LONDON — Negotiating’s easy, right? You play hardball, you don’t give any ground and if the other side look happy about the outcome then it’s probably because they’ve pulled a fast one on you. It’s all about winning.

This view of negotiation might nowadays be termed the Trump model — inasmuch as it’s a model at all. And it appears to be guiding Brexit negotiations, where the question tends to be: Who’s winning?

Partly, that’s because we often think about negotiations as being about money, which readily lends itself to this kind of haggling: My gain must be your loss, and vice versa. And partly, let’s be honest, people also just tend to be a bit competitive and think about themselves first.

But what people on both sides of the Brexit table are missing is that a negotiation shouldn’t be a competition, it should be an exercise in problem-solving. Put differently, you’re both on the same side.

A logical extension of that idea is that you have to care about “them” and what “they” want. The more you can get inside the other party’s head and understand what motivates their actions, the better the chances you’ll find an outcome that speaks to both your needs and theirs.

It also requires you to be more open about your thinking. It’s the very opposite of poker, which is why all the criticism in the U.K. about not “showing your hand” are so misplaced. In poker, secrecy is a fundamental requirement. In a negotiation, you’re trying to secure mutually advantageous outcomes.

And that’s perhaps the central difference.

Instead of seeing negotiation as zero-sum, the best negotiators see it as usually being positive-sum — by solving the problem, we can both gain for it. Not only does that approach encourage more cooperative behavior, it also drives better outcomes for both parties.

At the technical level, a lot of this is going on in Brexit talks. When it comes to the U.K.’s withdrawal agreement, the 80 percent that has already been signed off was built on this type of approach, with both sides scoping specific issues and seeking out areas of common ground.

But the remaining 20 percent is being held hostage by Trump-like tactics, with plenty of intransigence and less than full clarity about underlying intentions — especially on the British side.

To be sure, the British government finds itself in an unusual situation: It was handed a referendum decision and has had to work backward to construct a justification for leaving the EU, something that remains very much a work in progress.

With that in mind, the persistent desire of the U.K. to talk about the future relationship, ahead of the outstanding Article 50 issues, makes a certain amount of sense: Without knowing where you’re going, it’s rather hard to say how you’ll get there, or even which direction you should head off in.

But here’s the second major problem facing negotiators: Brexit isn’t a zero-sum equation. But it’s not a positive-sum exercise either — at least not in the short term.

Article 50 is about apportioning costs.

The U.K.’s withdrawal will necessarily inflict pain on both sides, be that through the transition to new arrangements or the necessarily less-comprehensive market integration that will ensue (no matter the type of deal).

Now, here you might argue that we should therefore be more Trump-like: Make the other side carry the can. And you certainly see echoes of that on both sides of the Channel, with talk of how it’ll hurt them more than us.

But that, too, is a blinkered view.

Article 50 isn’t a discrete and isolated exercise. It’s just one step in the ongoing relationship between the EU and the U.K. What happens now will shape what happens next.

Negotiators on both sides need to remember that finding a transparent and equitable way of sharing out the costs of the new relationship offers the best (or least-worst, at least) foundation for future discussions.

Using Article 50 to take advantage of the other side might secure some passing benefit — but will come at a long-term cost.

If the U.K. and the EU don’t brush up on better tactics in the next five months, they’ll both be poorer for it, both materially and politically.

Simon Usherwood is deputy director of the Economic and Social Research Council’s “UK in a Changing Europe” program and reader in politics at the University of Surrey, U.K.