I never meant to say that the Conservatives are generally stupid. I meant to say that stupid people are generally Conservative.
Joe says this is not a claim that most Conservatives are in fact stupid.
In logic, Joe's right. But in practice, he's not.
It all depends upon the proportions of Conservatives and stupid people in the population, and what Mill meant by "generally".
In the general election the year before Mill wrote those words, voters split 40-60 Conservative-Whig. Let's assume this represents the proportion of Conservatives in the population.
So, what did Mill think was the proportion of stupid people? Here's a clue from On Liberty:
On any matter not self-evident, there are ninety-nine persons totally incapable of judging of it, for one who is capable; and the capacity of the hundredth person is only comparative.
Let's put the weakest possible construal upon "generally" - just over 50%. It then follows that over 50% per cent of the population are stupid Conservatives. That's more than all the Conservatives in the 1865 election result; maybe some were too stupid to vote.
On this weak reading of "generally", the majority of Conservative voters in the 1865 election are stupid as long as more than 40% of all people are stupid.
On a a stronger reading of "generally" - three-quarters - then the proportion of stupids in the population has to be below 27% for the majority of Conservative voters not to be stupid. Given Mill's view of human intelligence, he probably didn't believe this.
One wrinkle here is that women and the working class could not vote in 1865. However, large numbers of these would have to be Conservatives to overturn my maths.
So, I reckon Mill did mean to say that Conservatives were generally stupid, and he was fobbing Sir John off, in the belief that, as a Conservative, he'd be too stupid to do the maths.