There used to be a pithy phrase about Unionists “killing Home Rule with kindness.” Yesterday, in the course of his first substantial speech on the constitutional question, the Prime Minister attempted to kill Scottish independence with emollience and a heavy dose of charm. It was, in short, the best articulation of the “case” for the United Kingdom I’ve heard from a Scottish or UK politician. The tone was pitch-perfect, but then that’s always been one of David Cameron’s strengths. His Edinburgh speech covered all the bases and anticipated likely criticisms; it targeted the head and the heart. Two “acknowledgements” were significant. First, Cameron acknowledged that the Conservative Party wasn’t “currently Scotland’s most influential political movement” and that “more than a little humility” was called for when any “contemporary Tory speaks in Scotland.” His argument for the UK, in other words, had little to do with party advantage. Secondly, the Prime Minister acknowledged that an independent Scotland could “make a go of being on its own, if that’s what people decide.” Often accused of implying that Scotland was “too wee” or “too poor” to go its own way, Cameron deftly closed down another likely line of Nationalist attack. The speech was also significant in that it acknowledged that Scotland’s devolved settlement has to progress beyond what’s currently on offer in the Scotland Bill. Although this moves the Conservatives into line with their Unionist partners, Labour and the Liberal Democrats, it directly contradicts Scottish Tory leader Ruth Davidson, who said last year that the Scotland Bill ought to be “the line in the sand.” Well yesterday the Prime Minister crossed that line, promising that “if” Scotland decided to remain within the UK, “then further options for devolution are on the table.” Cleverly, First Minister Alex Salmond likened Cameron to another Old Etonian, Sir Alec Douglas-Home, who made a similar pledge before Scots voted in the 1979 devolution referendum. Douglas-Home, of course, was not Prime Minister in 1979, and the notion that his intervention persuaded lots of Scots to reject devolution is fanciful. But therein lies an historical lesson for David Cameron: having made such a promise, the Prime Minister has to follow through or his credibility—in Scottish eyes—will be irretrievably compromised. Salmond also criticized Cameron for not providing any detail of these “further options,” which was a little rich coming from someone who wants to put “devo-max” on the referendum ballot paper while refusing to define it. The SNP, if they have any sense, will now be taking the Prime Minister seriously, but equally Unionists cannot rely on warm words alone. David Torrance is a journalist, broadcaster and writer. His unauthorised biography of the SNP leader and First Minister “Salmond: Against the Odds” (Birlinn) was published last year.