The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) classifies depression into the following types (there are even more, but these cover the basics):

1. Dysthymia. In essence, having a depressed mood on most days for at least two years.
2. Major Depressive Disorder. In addition to feeling "down" as in dysthymia, other characteristics may include excessive feelings of guilt and suicidal ideation, as well as various physical symptoms like loss of hunger and fatigue. It can be mild, moderate, or severe.
3. Adjustment Disorder with Depressed Mood. This is grief due to a loss of some kind (which itself can be classified as normal or complicated).
4. Depression NOS (not otherwise specified). Includes things like premenstrual depression and seasonal depression (SAD).
5. Secondary depression. Depression due to an underlying medical disorder like Cushing's disease or hypothyroidism.

Though not in DSM-IV, some practitioners further classify depression into two broad types:

• Endogenous (or chemical) depression to denote depression that arises without an obvious identifiable cause, thought to reflect some kind of "chemical imbalance" in the brain.
• Exogenous (or external) depression which is thought to arise from a specific, identifiable external cause.

Given this confusing and non-parallel classification scheme it's astonishing doctors don't become depressed themselves as they try to figure out into which bucket their patient's depression fits!

How can we make sense of all this and, more importantly, understand the real cause of depression in order to augment the effectiveness of currently available therapies?

MIND VS. BRAIN

First, we need to recognize the distinction between chemical and external depression has become outdated. Many neuroscientists have suggested that the mind arises from, and is actually caused by, the physical brain, meaning chemical and electrical reactions somehow give rise to thoughts and emotions. Evidence in support of this theory can be found in numerous studies that show altering brain chemistry with anti-depressant drugs (chemicals) can make depressed people feel better emotionally. The same is true for anxiolytics (like Valium) and their effect on anxiety.

But recently, with the advent of functional MRI scans (fMRI), we now have proof the opposite is equally true, that changes in thinking cause significant, measurable changes in brain chemistry and functioning. In one study, patients suffering from spider phobia underwent fMRI scanning before and after receiving cognitive behavioral therapy aimed at eliminating their fear of spiders. Scans were then compared to normal subjects without spider phobia. Results showed that brain function in patients with spider phobia before receiving cognitive behavioral therapy were abnormal compared to subjects without spider phobia but then changed to match normal brain patterns after cognitive behavioral therapy. This may represent the best evidence to date that changes made at the mind level are able to functionally "rewire" the brain, and that the brain and the mind are more mutually influential than we'd previously thought. It certainly supports the Buddhist view that brain and mind are in fact only two sides of the same coin, or different ways of viewing the same single thing.

DEPRESSION ALWAYS HAS A CAUSE

Where, then, does the true cause of depression lie? I would argue that depression arises at its core from a belief that we're powerless to solve our problems.

This is clearly true with people who know why they're depressed: invariably, once they figure out how to solve their particular problem, their depression lifts. But I would also argue this holds true for people who are depressed for no reason they know. Why? Because thoughts can trigger feelings that remain stirred up long after the thoughts themselves have been forgotten. Some studies have suggested people think upwards of 12,000 thoughts per day. How could we ever remember them all? Yet a fleeting thought we might have had this morning about the possibility of losing our job can and often does leave an emotional residue that lasts hours, days, weeks, or even longer. I would argue, therefore, any depression that appears to be "chemical" is more likely caused by a thought that simply isn't remembered—a thought about a problem we don't believe we can solve.

Further, sometimes what appears to be a "chemical" depression is caused by a thought that isn't directly or consciously recognized. These thoughts are often about problems that seem so unbearably awful and unsolvable we literally don't want (and often refuse) to think about them (such as our becoming jobless or the prospect of our own death).

Finally, I believe the commonly accepted idea that some forms of depression like depression NOS and secondary depression (#4 and #5 above) are caused by chemical or hormonal abnormalities overstates the case. I'd suggest an alternative explanation, that these forms of depression have a chemical or hormonal influence—reducing our ability to believe we can solve our problems but not entirely eliminating it. At first glance this might not appear to be a significant distinction given how incredibly difficult it is to believe in our ability to solve problems, for example, when experiencing premenstrual syndrome. But knowing intellectually we can win even if we're having a hard time believing it can help to sustain the most valuable thing depression tends to reduce: hope.

HOW CAN WE HELP OURSELVES?

None of this is by any means to say we can simply decide to believe we can solve a particular problem when no solution is obvious or forthcoming. Changing any belief, whether consciously recognized or not, is literally one of the hardest things to do. But armed with a clearer understanding of the true cause of depression we can consider the following steps to help ourselves: