There is a running joke among Western journalists, diplomats and other foreigners based in Iran who have the task of trying to understand what is going on behind the scenes: the longer you stay here, the more opaque Iranian policy making becomes.

It may be said lightheartedly, but it contains more than a grain of truth. The longer you spend trying to peel back the layers of the Iranian establishment to understand what the Supreme Leader Ayatollah Ali Khamenei is thinking, the more layers you discover.

And, frankly, as a Westerner — and even for Iranian journalists — there’s a very real limit to how many layers you are ever going to penetrate.

But penetrate you must because it’s Khamenei’s thinking that is the key.

President Mahmoud Ahmadinejad may be the most public — and often most worrying to Western capitals — voice out there. But he is just one of the layers. One constituency contributing towards consensus. When national decisions are taken, however, Khamenei will be behind them.

So determining Iran’s nuclear policy, the most sensitive of issues in the Islamic Republic, often seems to present more questions than answers. Does Iran want negotiations that will end the standoff with the West? Or is talking just a way to buy time to master nuclear technology? Has the establishment calculated that it can survive military strikes on its nuclear facilities? Or is it looking for the “red line” so it can pull back from the brink at the last minute? And, perhaps, one of the more worrying questions is: does the Islamic Republic know where that “red line” to prevent military action really is?

There are analysts who look at Washington and say, after more than a quarter of century without an embassy in Tehran, the U.S. ability to understand Iranian policy calculations has been deeply eroded. But the same too can be said of Iran, which under the shah was — Israel aside — Washington’s closest ally in the Middle East but now is a sworn enemy. Set together, the possibility that both sides will end up talking past each other is real.

Some analysts also describe a big gap between a U.S. policy approach that tends to want straight talking and the Iranian preference for pondering its path in drawn out negotiations. The stereotype again may not be so far from reality: the cowboy with his six-shooter versus the carpet seller in the bazaar working out a price over endless cups of tea.

So now, six world powers — the United States, Britain, France, Germany, Russia and China — have again offered a range of incentives, such as state-of-the-art civilian nuclear power technology and trade benefits, if Iran agrees to suspend uranium enrichment (Enrichment is the real worry to Western capitals because, despite Iran’s denials, they fear the process will be used to make nuclear bomb material not fuel for power plants).

We’ve been here before. The deal is not so very different from one offered in 2006, and which was roundly rejected by Iran. Some Western diplomats chatter that senior Iranian officials have been making more positive noises. Others are more sceptical. But everyone is wary of predicting which way it will go. And with good reason. It’s those layers, you see. To change the metaphor, what most of us following Iran are able to learn about the Islamic Republic’s decision-making are the outer ripples from a stone that has been thrown into a pond. What that stone looks like is well below the surface.