Elliptic pseudodifferential operator has at most finite number of non-positive eigenvalues

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Let $P \in OPS^m(M)$ ($m$ can be negative of positive) be a self-adjoint elliptic pseudodifferential operator on a compact manifold $M$. How to show that $P$ has only finitely many non-positive eigenvalues (in $L^2(M)$)? Does it somehow follow from the Gårding inequality?