That’s a rather philosophical question! I doubt there is a clear answer. In fact, I think you cannot separate the two: mental and physical. If we are not mentally healthy, this will manifest in physical signatures. For example, our hormone levels can change so that we have higher levels of stress-linked ones like cortisol if we are mentally unhealthy. Similarly, our levels of neurotransmitters that communicate messages between our neurons can change as well. So being mentally unhealthy has physical and biological symptoms that change cell function at a molecular level.
Equally, if we are physically unwell, this will have consequences for our mental health because we might feel stressed due to the lack of ability to do routine things. We might in fact seem the same hormonal and neurological stress responses as above. This raises fundamental questions – eg to what extent should we control mild variation in mental health, just the same as we do/don’t do for physical health?
That’s a hard one. The two are inextricably linked. When one goes wrong, the other tends to follow.
Often when someone is diagnosed with a serious illness, their mental health will deteriorate. If you are feeling depressed, it’s hard to motivate yourself to stay healthy and look after yourself.
If I had to pick one, I’d say mental health. It is much harder to hid a physical illness or injury than it is to keep a mental health problem a secret. You can usually spot if someone around you has the flu, but could you tell if they has schizophrenia? It’s much harder.
There is still a huge stigma attached to mental illness which can make it very hard for people to seek health. If you think you may have diabetes, you don’t mind discussing it with your friends, your colleagues, your pharmacist or your doctor. If you are feeling suicidal or think you may have bipolar disorder, it is much harder to come forward and talk to someone about it.
It’s important to be aware of both your own mental health and the mental health of those around you. Try and be understanding.
It doesn’t help that the human brain is massively complex and we still don’t fully understand it!