I still vividly recall hitch-hiking across Europe when I was 18 years old – the excitement over seeing all these great countries for the first time; the unique sights, sounds, smells, and not least, food that each destination had to offer. Endless possibilities within easy reach – breakfast in Amsterdam, lunch in Brussels, and dinner in Paris. Only the time spent waiting in long lines at the border crossings could dampen my wanderlust.

Luckily, we no longer need to worry about getting stuck at border crossings. Working together in the European Union, we've made it possible to zoom across borders with no passport – just as we have ensured the unimpeded free flow of goods, services and capital. That freedom has spurred European integration and made us the largest, wealthiest, and most open, economy in the world. And it has provided us with possibilities that would have been inconceivable just a few generations ago. My Irish daughter is a good example, having starred in the Belgian version of The Voice singing contest and studied in Denmark.

But with the growing threat of terrorism and rising refugee flows, we increasingly hear arguments for re-instating borders and building walls. The world is a dangerous place, so we're far better off closing our doors, the argument goes.

So, let's for a moment imagine Europe going back to the days of borders. Beyond putting European integration and cooperation at risk, the economic consequences would be disastrous. Currently, 1,7 million workers in the EU cross a border daily to go to work. According to the European Commission, border controls could cost commuters over 5 billion euros annually in terms of time lost. Overall trade could decrease by 20 percent and the tourism industry could lose as much as 25 percent of its revenue. All those numbers add up – a Bertelsmann Foundation study found that reinstating border controls would lead the gross domestic product of Europe to drop by at least 470 billion euros over a ten year period.

But how can Europeans feel safe if terrorists and criminals are able to cross borders as freely as anyone else? No doubt, the abolition of internal border controls should not come at the expense of security. So it becomes imperative that we jointly work to strengthen our external borders – keeping the wrong people out while facilitating access for those with legitimate and peaceful intentions to enter our EU territory.

With the unprecedented flow of refugees and migrants from the Middle East and Africa, we have put a raft of measures in place to do just that, including creating a European Border Coast Guard Agency and a European Counter Terrorism Center.

The intelligence community tells us that investing in counter terrorism cooperation is far more effective than running costly border crossings. Intelligence sharing between EU Member States reached an all-time high in 2016. And the Schengen Information System, used for border checks at the external border, has become an extremely valuable tool: its use jumped from 2 billion checks against the system in 2014 to almost 4 billion in 2016, leading to the arrest of over 25,000 people, the detection of over 79,000 people trying to enter illegally, and to finding over 12,000 missing persons. Mandatory systematic checks on all EU citizens against relevant databases at external land, sea and air borders are also now in place. And we just agreed to a new entry-exit system registering all third country nationals crossing the external borders.

Taken together, these are immense undertakings that send a clear signal: We will not let anyone or any threat force us to give up our borderless European society. We are determined to preserve our freedom of movement. Peace and prosperity flow from our open borders – closing them will only stop that flow.