What do we even sell? Well there's about £370billion of combined import/export trade with the EU every year, so I'd say quite a bit. They are our largest trading partner by a huge margin.
The ramifications of disrupted economic trade with the EU don't just stop with the companies which directly do the importing and exporting. It ripples through vast supply chains spanning many thousands of businesses and jobs.
The point is you cannot just disrupt this and expect British businesses to work it out without some serious damage along the way. We all run businesses, so we all know what it's like. A rise in import costs can wipe out a huge chunk of the profit margin for many businesses whilst making them less competitive on the world stage.
And we also all know that it's just not that easy to immediately switch to different suppliers in other countries, or to find new clients in other countries either. It has little knock-on effects everywhere which can accumulate into hundreds of billions of pounds worth of economic losses.
That's why you have the majority of SMEs, large businesses, multi-nationals, business bodies, trade unions and economists all believing that Brexit is a bad choice which will cause damage. Across all groups combined, they are HUGELY in favour of remain.
It's partly to do with existing trade, and partly to do with the potential for encouraging and increasing trade with new partners.
You could have said the same about Switzerland and China. Wouldn't China be happy to wipe import duty off if Switzerland did the same? Why not? Both countries benefit. No big deal, right?
Except the FTA Switzerland has agreed is that they will remove all tariffs for China now, and in exchange, China will remove tariffs for Switzerland in 15 years time. Plus, this includes some indefinite tariffs and quotas for certain products and services.
That's the thing about trade deals. The best case scenario for any country is to actually be allowed free trade in another country without the other country being able to trade in theirs. This allows the country to bring in external revenue without losing internal revenue when businesses start to spend more abroad.
Obviously, this cannot happen. FTAs are not a case of "let's find the fairest and best deal for both of us". FTAs are about concessions and compromises to reach a middle ground where both countries feel they have got the best deal for themselves.
This is why China demanded free access now in exchange for free access to their economy in 15 years time. China would have much preferred to give nothing if they could, but obviously Switzerland must get something in exchange eventually.
And Switzerland accepted that. Why? Because they need China more than China needs them, and with the largest economy by far, they have the leverage to dictate the terms.
If we leave the EU, and leave the world's second largest economy, China, the United States and many other large economies will do the exact same thing to us.
And if we leave with uncertainty over single market access, and years of potential negotiations to have to get through, we will not be in a position to just walk away. We will be pressured to accept what we can get.