  In this paper we introduce the prime index function
\begin{align}\iota(n)=(-1)^{\pi(n)},\nonumber \end{align} where $\pi(n)$ is the
prime counting function. We study some elementary properties and theories
associated with the partial sums of this function given
by\begin{align}\xi(x):=\sum \limits_{n\leq x}\iota(n).\nonumber \end{align}We
show that a prime $p>2$ is a twin prime if and only if $\xi(p)=\xi(p+2)$. We
also relate the prime index function to Cramer's conjecture by showing that
\begin{align}|\xi(p_{n+1})-\xi(p_n)|+2=p_{n+1}-p_n.\nonumber \end{align}That
is, Cramer's conjecture can be stated as \begin{align}\xi(p_{n+1})-\xi(p_n)\ll
(\log p_n)^2.\nonumber \end{align}This reduces the problem to obtaining very
good estimates of the second prime index function.
