Accordingly an event-particle could cover no other abstractive element.

This is the definition which I originally proposed at a congress in Paris in 1914[9].

There is however a difficulty involved in this definition if adopted without some further addition, and I am now not satisfied with the way in which I attempted to get over that difficulty in the paper referred to.
[9] Cf.

'La Theorie Relationniste de l'Espace,' _Rev.de Metaphysique et de Morale_, vol.XXIII, 1916.
The difficulty is this: When event-particles have once been defined it is easy to define the aggregate of event-particles forming the boundary of an event; and thence to define the point-contact at their boundaries possible for a pair of events of which one is part of the other.

We can then conceive all the intricacies of tangency.