Heidegger, a follower of Edmund Husserl's, "phenomenology", pointed out a fundamental flaw in dualist theories, and, consequently, in the whole mind-body debate.
Descartes' dualism is simply a consequence of a misleading vision of the world, according to which on one hand we have the "objective" world of "physical" reality (made of objects with physical properties) and on the other we have the "subjective" world of "mental" life (feelings, cognition, consciousness).
The physical world is described by some objective facts that do not depend on our existence. We can perceive those facts and think about them. And we can act in the world based on our thoughts. But our relationship to the world and its objects is detached, observer-like.
Basically, Heidegger reminded us that we are part of that world. We are one of its "objects". We don't exist as independent entities, we exist as part of the world. There is no way we can step back and watch what is happening in a detached manner: we are part of what is happening, and usually it is happening so fast that we don't even have time to think about it. We only have time to react by instinct.
Heidegger denies any value to the expressions "physical reality" and "mental life", and to the dichotomy objective/subjective: the world and the mind
cannot be separated. Everything is subjective, or objective (depending on the definition), as everything that we know is our "interpretation" of what is happening in the world and we have no way of having an "objective" interpretation of that happening because we are part of it.
In your daily life we do not adopt a detached, logic approach to situations but we just "act". Usually, you analyze one of your actions only "after" you have performed it; and, typically, this happens only when something went wrong: you pause to reflect and analyze what and why went wrong. Most of the time we are not "conscious" of why we are doing what we are doing.
Heidegger says that we are "thrown" into the world. Normally we don't "break down" the situation: we "break down" the world around us only when our actions fail and we need to find out why.
For example, we don't normally realize consciously what tools we are using to perform an action: a pair of scissors or a glass or a clutch stick. Only when our action fails, we focus on the tool that we are using and why it is failing us.
When we are hammering a nail into wood, we are not interested in discussing the properties of the hammer and the nail and the wood, we just hammer. If it doesn't work, then we stop and analyze what is wrong with the hammer, the nail or the wood.
Same with the objects that surround us: we are rarely aware of every single object that surrounds us. But let's say that somebody locked us in a room and we need to find a way out of there: only then we will "break down" the reality of that room into all of its object, desperately looking for one that will help us.
Sometimes when you suddenly focus on the drive to work, you get lost: all of a sudden, you don't recognize anymore the streets that you drive through every single morning. There are so many details that you never noticed: was there really a curve? is there really a billboard in that curve? And so forth. But if you don't focus on the route, you know perfectly well how to get to work.
In everyday's life, we do not have a complete representation of the situation, and we cannot predict all the consequences of our actions; and we do not have time to search for either the representation or the prediction. Nonetheless we understand a situation and we act in it. And most of the times we survive. Only when our actions fail, do we need to step back and analyze the situation and try to figure out rationally why we failed. Logic is something that we use "after" the fact, to "debug" what we did wrong.
The science that we built to analyze the world is a complication. The truth is much simpler and so closer to our ordinary life.
There is a fundamental unity of the "Dasein" (of being). Subject and object cannot be separated. They cannot exist independently.
An individual is not a separate entity but a manifestation of Dasein in a world and within a tradition (society is a big component of that world).
We cannot study out beliefs as objects because we cannot abstract from them and observe them objectively. They are part of our belief system and every action we perform is affected by that same belief system, so we enter a vicious circle. We carry a burden of experience and knowledge with us which shapes our actions.
When we study something rationally, in a detached way, we are actually missing something by isolating it. Understanding something is being part of it. Cognition is praxis. We are "thrown" in the world and that's how we understand it and act in it. If we stop and observe it, then we may not be as good at acting in it.
Of course, Heidegger does not need mental representations to reason about. What matters is action: action of the world and action of us in the world. Representation is interpretation. There is no "objective" fact (or absolute truth) about the world.
Not to detract from Heidegger, but
it is ironic that Heidegger, the one who told us that Science was making simple things difficult, is today remembered as being one of the most difficult philosophers ever.
The truth is that Heidegger's theory sounds obscure because it is so unscientific and we have trouble understanding unscientific theories. That's why we invented science: to explain things that appear beyond comprehension.
Luckily for most of us, the people who design bridges and cars do not follow Heidegger's philosophy: they step back, analyze the situation, carefully compute natural laws of the objective physical world and then build artifacts that do work.
Click here to read a review of "Being and Time" (1962) , not my favorite book despite its influence.