I began reading it with a hopeful and anticipatory attitude, and was pleased to find that the author invokes the metaphor of the mind as a garden early in the text. I think that's a great metaphor, and have been enjoying it for years. However, as I read on, the full meaning and extent of the author's idea became clear and I became seriously annoyed.
He takes a very hard stance: He proclaims that a person's fate is entirely their own doing, and that anyone who whines about circumstances is exercising an inefficient or impure practice of thought that serves only to damage their hope of improvement. In a nutshell, his advice should boil down to, "Whining never solved anything." (Which is not entirely true, since whining is what we in the software development and I.T. worlds call "user feedback", and we listen to it and sometimes even employ it ourselves as a way of prioritizing problems.)
But anyway, "Whining never solved anything" is a reasonable attitude around which to structure one's thoughts. Sometimes at work I am amazed by the sheer energy people can devote to whining at the wrong person, letting off emotional steam but doing nothing to solve their problems because the person who needs to hear their feedback is the one person they don't have the gumption to hand it to. If you sit down and think about it, especially about the exchanges you have with other people or the internal monologues you create, it's sometimes startling how much of it can be boiled down to "whining", and eliminated without any appreciable loss of quality. There are conversations - most of which are had in engineering departments and in bedrooms, I suspect - where people squelch their egos in the orderly pursuit of solutions to the problems they have in common, but the clarity of even those conversations is always tainted by some hint of a power struggle, deferred or disguised, waiting for the opportunity to spring up like a cheap jack-in-the-box, and indulge our human need to subjugate our competitors, nitpick, and broadcast our fetid complaints. Keeping that fellow stuffed in the box while still addressing the problems that inspire him to pounce is our main occupation as peace-loving problem-solvers.
I'm getting a bit histrionic here, so I'll move on. This "Whining isn't constructive" idea is alright, but it's not what irritated me about "As A Man Thinketh". It's the way Mr. James Allen extrapolated his idea, rolling it out farther and farther like a rotten oriental rug until it covered territory it had no business being in. If a person is poor, he says, then that person has earned their financial destitution purely through the sinful habit of ungoverned thinking. If a person is sick, then they have invited disease into their bodies through poisonous thoughts. Not just sick, in fact, but sickly - as in, short, weak-muscled, or nearsighted ... all of these conditions are the result of "sinful thinking". He pumps his idea up to be so non-deterministic that it inverts itself, and becomes deterministic in practice. Imagine some poor chap in the 1800's (when this book was written) who has bowel problems due to an extreme food allergy, and reads "As A Man Thinketh" and is convinced that if he just changes his mental character, the malady will burn off like so much morning fog ... and then ten years of suffering down the line he dies from colon cancer, his fate just as securely determined not by his environment, but by his refusal to investigate environmental factors.
That's why this book falls flat and irritates me. The power of the mind is not absolute. Nor does the mind act as the sole source - the "seeds" - of it's own evolution. The truly useful idea is neither internal nor external: You and your environment act to shape and reflect one another. Therefore, consider the world as an extension of the content of your mind. Determine the thoughts and attitude you wish to cultivate, and then act to build, or locate, the environment where those thoughts can be put to use. ... where it is most beneficial to you when that attitude is indulged. That's a pursuit which requires flexibility, a consistent open state of investigation, and favors self-improvement through the most concrete means possible.
It also, unlike "As A Man Thinketh", doesn't insinuate a subtle contempt for individuals less successful than oneself, because it acknowledges the intimate connection we each have to our own past and present environment. A child indentured to a textile mill and a man drafted into a foreign war have legitimate complaints. I thumb my nose at James Allen and his Christian Science.