Imagine for a moment that Jeremy Corbyn or Nick Griffin sought to join the Conservative Party. Both would be barred. And no real conservative or Conservative would complain. The line must be drawn somewhere. There must be some quality control.

But those two gentlemen notwithstanding, it remains true that defining conservatism – and therefore Conservatives – is peculiarly riddling. Unlike socialism, national or international, conservatism is less a creed than a disposition, less international than local, less the product of belief than custom. A socialist in one country will share an ideology with one in another. By contrast, conservatism differs hugely even between neighbours. A British and French Conservative are very different animals. The conservatism of the former was given foundational expression by an Irishman, Edmund Burke. Ireland today has not one but two centre-right parties. Neither would describe themselves as conservative.

These abstract musings have a concrete point. There is current debate about infiltration of the Conservative Party.  Publicity-seeking former UKIP donors, witch-hunting Remainer obsessives, media outlets in search of August stories, the Conservative Party’s electoral opponents: all have an interest in talking up the claims. Our own enquiries suggest that there is no mass joining-up by new members. But even a limited rise, given the context, raises the i-word – or rather questions about it.

Does it hold automatically that former UKIP members are not potential Conservatives, for example? Should the Party really seek to ban recruits from its right as well as from its left?  And if so, why? If the Party really is a broad church, it will be open to all who share the bundle of broad attitudes to which we refer above. Obviously, that excludes the Griffins and Corbyns, the racists, the Muslim-haters and the anti-democrats. But a big section of UKIP supporters fall outside those grounds – unless, that is, one is to take the same terrifying stance as Rotherham social services.

None the less, here is an irrefutable case for barring those who seek to join the Party as part of an organised would-be putsch from outside.  Jacob Rees-Mogg was absolutely right to warn recently against “single issue fanatics” – people who identify so strongly with one element of contemporary conservatism as to have no interest in any of the others.  But how would any vetting process of a Tory Momentum be exercised? CCHQ has no machinery for exercising loyalty tests – having traditionally treated them as incompatible with the spirit of inclusion that should mark any mainstream centre-right party. It is simply not set up to deal with any organised infiltration programme.

This takes us to the heart of the matter. There is a vogue for arguing that modern parties can’t recruit a mass membership. William Hague came very close to arguing this recently in his Daily Telegraph column.

If so, it follows that they really will be vulnerable to infiltration – because there will eventually be no established body of members greater in number than the newcomers. Or else, if those infiltrators are resisted, the party in question will have a barebones membership. And those would-be infiltrators will pour instead into a new electoral force.

In short, a party without members is a party without a future. Just as the National Trust couldn’t display houses to the public without members and volunteers, so the Conservative Party won’t win elections without activists and supporters – and become a rooted local presence in our national political culture.

The great Tory infiltration row is precisely the wrong way round. The Conservative Party’s main problem isn’t being infiltrated by the wrong members. It is having too few of the right ones in the first place. And too many people near the top who believe it can get on just fine without them.