  The classical Eckmann-Hilton argument shows that two monoid structures on a
set, such that one is a homomorphism for the other, coincide and, moreover, the
resulting monoid is commutative. This argument immediately gives a proof of the
commutativity of the higher homotopy groups. A reformulation of this argument
in the language of higher categories is: suppose we have a one object, one
arrow 2-category, then its $Hom$-set is a commutative monoid. A similar
argument due to A.Joyal and R.Street shows that a one object, one arrow
tricategory is `the same' as a braided monoidal category.
  In this paper we begin to investigate how one can extend this argument to
arbitrary dimension. We provide a simple categorical scheme which allows us to
formalise the Eckman-Hilton type argument in terms of the calculation of left
Kan extensions in an appropriate 2-category. Then we apply this scheme to the
case of $n$-operads in the author's sense and classical symmetric operads. We
demonstrate that there exists a functor of symmetrisation $Sym_n$ from a
certain subcategory of $n$-operads to the category of symmetric operads such
that the category of one object, one arrow, . . ., one $(n-1)$-arrow algebras
of $A$ is isomorphic to the category of algebras of $Sym_n(A)$. Under some mild
conditions, we present an explicit formula for $Sym_n(A)$ which involves taking
the colimit over a remarkable categorical symmetric operad.
  We will consider some applications of the methods developed to the theory of
$n$-fold loop spaces in the second paper of this series.
