%Whether it is about food, energy, or interaction, the word \emph{natural} suggests the idea that the object it refers to is good for people, their health, or the environment for example.
%It is the idea that what was made by the nature is better than artificial artifacts, which are made by humans.
The first question is: what do we mean by “natural”?
-It primarily refers to waht exists in the nature.
+It primarily refers to what exists in the nature.
However gestures are not objects that can be found in the wild.
It also refers to what comes to mind, or prior skills.
People have skills in manipulating physical objects that we can leverage in interactive systems.
Yet, people have different backgrounds, skills, and culture.
Therefore what comes to somebody's mind is not necessarily the same than what somebody else is thinking.
The second question is: is it better because it is natural?
-This is a vast question because it depend on what metric we would like to optimize: performance, learnability, guessabiity, etc.
+This is a vast question because it depend on what we would like to improve: performance, learnability, guessability, etc.
There is no general guarantee that gestures improve any of these measures.
+They have to be carefully designed for this.
+This is the reason why decades have passed between the first gestural interaction systems and the first successful commercial products based on gestural interaction.
Norman discussed the concept of NUIs and concludes that what matters is not whether these interactions are natural or not~\cite{norman10}.
-What matters is that it is an alternative interaction modality for the design of interactive systems, and the same usability rules apply.
+What matters is that it is an alternative interaction modality for the design of interactive systems, and the same usability rules apply to them.
Now, if the natural essence of gestural interaction is not an essential benefit for the design of interactive systems, we can think about this modality differently.
%The physical world has constrains that the digital world does not have.
We can move through objects or even fly.
We can manipulate objects remotely, and independently to their weight, size, or shape.
Hence, when when designing interactive systems with gestural interaction there is no necessity to reproduce the physical world.
-We should rather focus on what we would like users to achieve, and optimize metrics such as performance, learnability, and so on.
+We should rather focus on what we would like users to achieve, and improve performance, learnability, and so on.
On a practical point of view, we care about the input vocabulary, the properties we would like to manipulate in the virtual environment, and the mapping between them.
%mapping between DOFs and objects manipulated
For example, in the section~\ref{sec:interactionvr} we present two interaction techniques for immersive virtual environments.
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