It's hard to think of another foundational text, especially in a field with as much pretense to objectivity as economics, that is so personal and so suffused with the author's own feelings and state of mind. Of course the book is systematic in a way, trying to create the image of a whole economy that revolves around concepts like the marginal efficiency of capital and liquidity preference, but much of it is filled with stray strands of logic or evidence, often profound, always thought-provoking, that are meant to jar the reader and give one a sense of even more radical possibilities.
I am grateful that before diving into this book I spent time reading the background to Keynes's thought, in both his own, earlier works, and in syntheses like those by Skildelsky and Laidler. This book, after all, is very much of it's time. It tries to explain both the dire situation in which the world found itself in 1936, and the history of what Keynes felt was the false thought that brought them there. One surprising thing about the book, in fact, is how much of it is taken up with a history of economic thought itself. Keynes basically traces the errors of their age to David Ricardo, who refused to listen to his friend and epsitoler Thomas Malthus, who pointed out that in a money economy there could be a "general glut" of products without purchasers. Ricardo denied this and therefore refused to believe that the output of the economy at any one time was not the maximum possible.
Keynes, as always exaggerating a little, pointed out that contemporary theories of the "trade cycle" were based on Ricardo's ideas of constant, maximum employment, and therefore were contradictory. Keynes thus brought his own, sturdy psychological tools to bear on the issue, and tried to show how a confusion about the future, combined with an excessive love of money, ensured that economies could long be stuck at a unnecessarily low level of output. His argument, in a nutshell, is that entrepreneurs cannot know the future, but their current production is based on it, so output is determined by how entrepreneurs feel about their future profitability (what he calls the "marginal efficiency of capital," basically a psychological concept). If entrepreneurs become unduly pessimistic (they lose their "animal spirits") they are less liable to produce goods and thus create jobs and demand, and therefore their own pessimism creates the low profitability they were worried about.
Other economists at the time claimed that meant the entrepreneurs just saved their income, and this saving provided more money for loans, which reduced the rate of interest on them to a point that it would be profitable for the entrepreneurs to borrow and produce again. Keynes, however, pointed out that loans don't necessarily come from savings, people could "save" under their bed or in cookie jars, and this preference for cold, hard cash (what Keynes called "liquidity preference") meant sometimes savings was simply wasted or not invested. Therefore, interest rates could be higher than the marginal efficiency of capital and production could sag, perhaps for decades or even centuries. In those cases, the government needed to lower the rate of interest or create more investment and demand to energize entrepreneurs to produce again.
These are the basics of Keynes's argument, but such a sketch hardly does the book justice. Much of it is a meditation on the nature of economic thinking, about how individual human desires get transmuted into collective action or inaction in an economic system, and about how the human urge for security can sabotage itself. In the end, the book, by the man who once strove to be a philosopher, is a call for carpe diem, both in taking risks and enjoying the fruits of one's efforts. It contrasts this virile public and private attitude with the dour, Calvinist miserliness that Keynes saw as inherently destructive of happiness. It's no coincidence that his economic formulas mimicked his own, personal attitudes. Keynes, himself an intellectual biographer, understood this connection perfectly.
One can point out many places where contemporary economic thought has superseded Keynes. The book's misunderstanding about potential divergences of savings and investment, and its ironic inability to model a dynamic interaction of the two, is widely acknowledged. Likewise, Keynes's simple-minded belief that prices rose only at maximum employment has been discredited. Keynes seems especially confused about "capital," not sure whether he means by it stocks and equity or actual, physical investments. Still, many of his other insights have never been fully incorporated by economists of any stripe. The "rational expectations" revolution has pushed away much of Keynes's emphasis on animal spirits, but what is perhaps surprising here is how much attention Keynes gave to expectations in general, and their inherent instability. Like so much of the book, these insights are difficult to incorporate in a simple model, but they remain for future thinkers to ponder and wonder.