The question of how sensory perceptions arise and what role they play in our experience and development has occupied many. Aristotle, for example, sees the ability of sensory perception as distinguishing between animals and plants. If an animal is to grow up, live, and reproduce, it must be able to find its way in the world. The ability of perception serves this purpose. Accordingly, various sensory organs are formed in living beings to enable perceptions.
The philosopher, Emanuel Kant emphasizes the sensual perceptions in their interaction with the intellect. In a 'critical' reflection he describes an approach on how to grasp from this, the 'conditions of possibility' of human experience. Both approaches have occupied many deep thinkers, from the Middle Ages right through to the present day, and led to a high degree of consideration, reflection and valuable insights.
A central question remains however: how do our perceptions become the basis of our experiences? The phenomenon of perceptual illusion and hallucination is also an important question here. If these types of error are possible, how reliable are our direct perceptions of the world? 
Perception, Intellect, and Intention
Our perceptions are initially based on the activity of our sense organs. With their help, we take part in the external world. However, other factors besides the sensory organs also play an important role in the emergence of perceptual content.
This can be illustrated by the example of a game. Various famous buildings are shown on a projector, such as the Statue of Liberty, or the Eiffel Tower. At first, the projector is out of focus, and only vague color impressions can be seen without clear outlines. Gradually, the image is made sharper. The first person to recognize the building wins. This example shows how the mind and memory combine the sensory impressions into a whole. Only in this way a connection between the impressions arises and one recognizes the building. Kant calls this process the 'synthesis of the manifold'.
One can assume that this process also takes place in animals. For them too, perception helps them to identify other objects in their environment. In humans however, the process is differentiated more acutely by the interaction between conceptual thinking and language.
This process also plays an important role in technical image recognition: the individual data points of a digital camera are processed by various technical functions in such a way that objects can be recognized, for example letters on an envelope, or objects in road traffic.
But it is not only contents of memory and mind that flow into our sensory perceptions and influence them. Other mental contents also play a decisive role here. The perceptions arise at the intersection of the senses and consciousness. Our desires or fears have a corresponding influence on what we see. In everyday life we sometimes speak of seeing only what we want to see, or only that for which one is sensitized, even before the perceptual experience.
The spontaneous connections and interpretations that take place in every perceptual process require a 'pre-understanding', that is, a pre-existing idea, a pre-existing concept. Our emotional states and our intentions play a central role in this. This is important in many contexts in order to be able to react quickly. The classification of perceptions in road traffic has to be fast, and the corresponding conditioning enables us to make quick decisions.
But there is also 'slow' thinking alongside this fast thinking. Especially when it comes to perceiving things more deeply and differently, conditioning can hinder us. Impartiality and openness are important prerequisites for new perceptions and insights. Therefore, it can be important to open up to constructive provocations, to become aware of one's own perspectives, to question them and to let them go.
The ‚Fourth State‘
Our sensory perceptions are the basis for our experiences in the external world. However, they are only one 'mode' of human consciousness. For example, four states are described in the Mandukya Upanishad:
‘Brahman is all, and the Self is Brahman. This Self has four states of consciousness. The first is called Vaishvanara, in which One lives with all the senses turned outward, aware only of the external world. Taijasa is the name of the second, the dreaming state in which, with the senses turned inward, one enacts the impressions of past deeds and present desires.
The third state is called Prajna, of deep sleep, in which one neither dreams nor desires. There is no mind in Prajna, there is no separateness; but the sleeper is not conscious of this. Let him become conscious in Prajna and it will open the door to the state of abiding joy. […]
The fourth is the superconscious state called Turiya, neither inward nor outward, beyond the senses and the intellect, in which there is none other than the Lord. He is the supreme goal of life. He is Infinite peace and love. Realize him!
Here, a different perspective is taken. The starting point is the statement that the self and Brahman are one. As living beings, we are part of nature and, similar to our relatives from the animal kingdom, we are equipped with sense organs that have developed over long periods of time, and that allow us to participate in the external world.
But besides this - according to the Mandukya - our core is one with the supreme reality. In this view it is logical that, besides the sensory perceptions, their structuring by the mind and memory, besides the shaping of experiences by intentions and apprehensions, a state is sought in which we connect with the innermost core.
The Upanishad names two forms of sleep in addition to the waking state. Modern sleep research distinguishes between the so-called REM phases (REM=Rapid Eye Movement) and NREM phases (non-REM phases). A REM phase is a stage of sleep characterized by rapid eye movements, increased heart rate, intense breathing and vivid dreaming. Brain activity (measured as EEG) resembles that of the waking state.
It is natural to relate this phase to the dream state of the Mandukya, and the NREM phases of deep sleep to what is called ‘dreamless sleep’ in the Upanishad (although, according to current understanding, dreams also take place in this phase, but they are in fact quite different in nature from those of the REM phases).
In the understanding of the Upanishad, in the state of deep sleep we do experience the dissolution of separation, but this occurs without the sleeper being aware of it. In the 'fourth state' – Turiya – however, it is precisely about awakening in a fully conscious state. This is described as the highest goal of life, the attainment of pure consciousness.
At the same time, this pure consciousness is not separate from the other states, in particular not from the sensory perceptions. Rather, the fourth state underlies and permeates the other three states of consciousness. The more the 'door to abiding joy' is opened, the more the other states will share in it. One who awakens in Turiya, also sees external things with new eyes.
What has been said opens up a very far-reaching perspective. Such a perspective is not only found in the Indian tradition. With the Greeks, one could speak, for example, of the 'dream ride' of Parmenides.  In Western mysticism, similar thoughts can be found, for example, in Meister Eckhart's work. And there are certainly numerous other references in other cultures and eras.
This is interesting in itself, but even more immediate is the question of whether and how we experience any of it. Do we know that stillness, that hesychia  in which our sensory perceptions come to rest, in which we can let go of memories, desires and fears, in which we create a space of silence and openness? In Turiya we dive into a silence that is the basis of all true dynamism, into a depth where we find ourselves. From this depth, the Upanishad finally pronounces the threefold 'Shanti' (peace) that permeates the other three states.
 De Anima ii 3; De Sensu 1
 Kant, Immanuel (1781): Critik der reinen Vernunft. Riga.
 Crane, Tim; French, Craig (2021): The Problem of Perception. In Edward N. Zalta (Ed.): The Stanford Encyclopedia of Philosophy. Fall 2021: Metaphysics Research Lab, Stanford University
 Kahneman, Daniel (2012): Thinking, fast and slow, London
 Easwaran, Eknath, The Upanishads, Nilgiri Press, p. 204
 Brockhaus, Schlaf. http://brockhaus.de/ecs/enzy/article/schlaf-20 (aufgerufen am 2022-02-05)
 Kingsley, Peter (2001): In the dark places of wisdom, London
 Kingsley, Peter (2004): Reality, Point Reyes
 Steiner, Rudolf (1901): Die Mystik im Aufgange des neu-zeitlichen Geisteslebens und ihr Verhältnis zur modernen Weltanschauung. 3. Auflage (2009): Rudolf Steiner Online-Archiv
 Personification of stillness in Greek mythology