Definition:
The ardt is the first of the Four Core Notions, and like all the Core Notions, there is not an exact English equivalent. The best translation is 'inner-self' or 'soul' since it refers to the 'you' within you. But it is also the heart, and the mind.
It is not so much the intellectual mind, but rather the deeper self: the emotional and spiritual mind (hence 'heart'/'soul').
So it is the deeper emotional, mental, and spiritual self within us. It is essentially the combination of the heart, mind, and soul. It is the inner-self.
A good English equivalent would be 'psyche', however this has connotations which carry over from psychology and psychoanalysis. Kibtisk thought does not break the ardt down into Freud's id, ego, or superego. It is does not delve into the subconscious archetypes or shadows of Jung. When referring to this kind of mind the Kibtisk language uses a different word (myndt). So whilst 'psyche' might be a good equivalent, it does not come without preconceived notions inapplicable here.
The ardt then is simply the inner-self: that which is within you through which you spend your life navigating. It is the earth you journey (yürna) across.
Etymology:
The word ardt comes from the Kibtisk word 'yarda', which means 'earth'.
This is because, like earth, the inner-self can be hard or soft, cold or warm, barren or fruitful, empty or full. What makes it so are the external elements: the weather (ihre) and the seasons (circumstances).
Additionally, as mentioned above, the ardt is the earth we journey across, and the state of the ardt, as well as the weather (ihre) and seasons (circumstances) all affect us on our journey of life through the ardt (known as the yürna).
Explanation:
This journey through the ardt is a conscious effort and a key practice of the Kibtisk Way. Followers of the Kibtisk Way will make an intentional effort to gauge where they are within themselves through a process called ardzýn - literally seeing the ardt (zýn = to see). In English ardzýn can be literally translated as 'soul-seeing' or more generally 'self-knowledge'.
How this is done is discussed in the section on Kibtisk Practices, but the basic idea behind this concept is that of self-knowledge and self-awareness; or more accurately inner-self-knowledge and inner-self-awareness. Being aware of our inner-selves: our emotional, mental, and spiritual states, allows us to navigate our way through life in a way that benefits us and others.
Practising ardzýn - seeing the ardt - is the equivalent of surveying the landscape which we are journeying over in order to know where we are and plan a route to where we want to be. By being aware of our inner-selves and our trajectory we can guard what we baden. We can be aware of atmospheres (ihres) which hinder or benefit our internal progress and intentionally avoid or embrace them, as well as being aware of what we are projecting from within onto the atmosphere and people around us.
Therefore, the ardt is a Core Notion of the Kibtisk Way because it concerns our very selves and the very earth beneath our feet as we wander on this life-journey.