|Left to right. Samuel Johnson c. 1772, painted by Sir Joshua Reynolds. “Am I not a man and a brother” – medallion made in 1787 by Josiah Wedgwood . James Boswell at 25, by George Willison|
The first thing to say is Johnson was a strong slavery abolitionist, and no friend to the American colonists. Boswell records that in 1777, when in company with some very grave men at Oxford, Johnson’s toast was, “Here’s to the next insurrection of the negroes in the West Indies.” Johnson’s “violent prejudice against our West Indian and American settlers appeared whenever there was an opportunity”, Boswell tells us, revealing his own prejudice. Of the American colonists, Johnson said: “how is it that we hear the loudest yelps for liberty among the drivers of negroes?” 
His colour sufficient testimony
It seems that in the year 1777 a negro was claiming his liberty in a Scottish court, and Johnson dictated an argument in his favour. No law but that of violence, subjects a negro to his master, he argues; and the slaveholder’s pretended claim to the negro’s obedience is based on having “bought him from a merchant of slaves, whose right to sell him never was examined …. The laws of Jamaica afford a Negro no redress. His colour is considered as a sufficient testimony against him.” The argument is worth reading in full, and I have it for you, along with a handful of other extracts from The Life of Johnson, on a separate page. Johnson could get vexed when opposed in argument, and after debating slavery and the taxing of the American colonies, two subjects which Johnson and Boswell disagreed on, they went to bed bad friends.
As you would expect from the writer of the first dictionary, Johnson was jealous of infractions on the English language. He found fault with Boswell for using the phrase to make money. “Don’t you see (said he) the impropriety of it? To make money is to coin it: you should say get money.” Boswell doesn't agree though, and thinks the phrase to make money is pretty current. In an object lesson to those of us who would stem the tide of language change, Johnson “was particularly indignant against the almost universal use of the word idea in the sense of notion or opinion, when it is clear that idea can only signify something of which an image can be formed in the mind”. We may have an idea or image of a mountain, a tree, or a building; but Johnson objected to an idea or image of an argument or proposition. Lawyers “delivering their ideas upon the question under consideration” was modern cant, he thought.
The finest head cut on a carrot
Going into a convent for fear of being immoral was like a man cutting off his hands for fear he should steal. “There is, indeed, great resolution in the immediate act of dismembering himself; but when that is once done, he has no longer any merit: for though it is out of his power to steal, yet he may all his life be a thief in his heart.”
He argued against the value of sculpture. Painting is okay, as it consumes labour proportionate to its effect; “but a fellow will hack half a year at a block of marble to make something in stone that hardly resembles a man. The value of statuary is owing to its difficulty. You would not value the finest head cut upon a carrot.”
Two final offerings, Johnson declared “It is commonly a weak man who marries for love”, and thought it was better to shoot a highwayman in the heat of the moment than to testify against him later in cold blood. You can find all these things in my extracts.
I'm collecting books that Jane Austen had on her own shelves. According to her brother Henry, Johnson was her favourite moral writer in prose . And in a 1798 letter, Austen wrote of getting Boswell’s Life of Johnson. I'm now reading Johnson’s Rasselas, and have recently finished a handful of other 18th century novels. Which are not, I'm happy to say, as bad as I expected. My mistake was starting with Richardson’s Pamela. It's dire, but I'm reassured to find Johnson also thought Richardson dire. More of this anon perhaps.
As a postscript, I see that challenged by Boswell about his prejudice against the Scots, Johnson admitted: “Why, I own, that by my definition of oats I meant to vex them.” (1783)
 For slave medallion by Staffordshire pottery manufacturer Josiah Wedgwood, see Smithsonian National Museum of American History
 In Rasselas I find: “Knowledge is certainly one of the means of pleasure, as is confessed by the natural desire which every mind feels of increasing its ideas.” (ch XI) Here Johnson appears to have flouted his own rule.
 Henry Austen “Biographical Notice” in the 1st edition of Northanger Abbey (Dec 13th 1817)