“Adobe claims that we are a closed system, and that Flash is open, but in fact the opposite is true. Let me explain.”

So begins the meat of Steve Jobs’ essay on Adobe (ADBE) Flash — the first extended piece of writing we’ve seen from Apple’s (AAPL) CEO since his Thoughts on Music in Feb. 2007 and A Greener Apple three months later.

Thoughts on Flash runs nearly 1,700 words and is sure to be carefully parsed in the days ahead. It’s basically a five part expansion of the line his PR people began putting out a few weeks ago. He argues:

• Flash is 100% proprietary, unlike the open standards Apple supports, like HTML5.
• Adobe’s claim that 75% of video on the Web is in Flash is misleading. “What [Adobe doesn’t] say,” he writes, ‘is that almost all this video is also available in a more modern format, H.264.”
• Flash has a poor security record and the No. 1 reason that Macs crash. “We have been working with Adobe to fix these problems, but they have persisted for several years now. We don’t want to reduce the reliability and security of our iPhones, iPods and iPads by adding Flash.”
• Flash performs particularly poorly on mobile devices because it uses software decoding, rather than hardware decoding — thus draining battery life.
• Flash wasn’t designed to work — and don’t work well — on touch-based interfaces like those that drive Apple’s latest products.

And then he gets to the heart of the matter:

“We know from painful experience that letting a third party layer of software come between the platform and the developer ultimately results in sub-standard apps and hinders the enhancement and progress of the platform. … We cannot be at the mercy of a third party deciding if and when they will make our enhancements available to our developers.”

UPDATE: The Wall Street Journal gave Adobe CEO Shantanu Narayen a platform to respond in the form of an exclusive interview with Alan Murray. The Journal‘s live-blog of its own event is available here. A “Flash Fight” video with Journal commentary can be seen here.

See also:

• Has Steve Jobs gone mad?
• Why Is Steve Jobs Flash-obsessed?

[Follow Philip Elmer-DeWitt on Twitter @philiped]