Although the Conservative prime minister grasps this, one wonders how many of the Labour leadership contenders really do too?
They mouth platitudes about regaining the trust of voters, but there is nothing to suggest that any of them are prepared to look deep into their own and their party’s soul and accept that there is something fundamentally indecent that needs to be expunged.
All the candidates who have declared so far – Sir Keir Starmer, Emily Thornberry, Clive Lewis, Lisa Nandy and Jess Phillips – keep talking about unity.
Sir Keir, the early favourite, spoke at his launch about the need to “put factionalism and division to one side and unify around a radical programme”.
Even Mrs Phillips, whose message is that she is the only candidate who truly connects with ordinary voters, has spoken of the need for the party to come together.
It is all very warm and comforting.