\node (a) at (-2.5, 2.5)   {a};
\node (b) at (2.5, 2.5)    {b};
\node (c) at (-2.5, -2.5)  {c};
\node (d) at (2.5, -2.5)   {d};

\node (w) at (-1, 1)  {w};
\node (x) at (1, 1)   {x};
\node (y) at (-1, -1) {y};
\node (z) at (1, -1)  {z};

\draw[->] (a) -- node[midway] {f} (b);
\draw[->] (b) -- node[midway,right] {q} (d);
\draw[->] (a) -- node[midway,left] {p} (c);
\draw[->] (c) -- node[midway,below] {g} (d);

\draw[->] (w) -- node[midway,below] {h} (x);
\draw[->] (x) -- node[midway,left] {j} (z);
\draw[->] (y) -- node[midway,above] {k} (z);
\draw[->] (w) -- node[midway,right] {l} (y);

\draw[->] (a) -- node[midway] {$\upsilon$} (w);
\draw[->] (b) -- node[midway] {$\phi$} (x);
\draw[->] (c) -- node[midway] {$\chi$} (y);
\draw[->] (d) -- node[midway] {$\psi$} (z);

\node (wxyz) at (0, 0)    {$\kappa$};
\node (awyc) at (-1.8, 0) {$\lambda$};
\node (awxb) at (0, 1.8)  {$\mu$};
\node (bxzd) at (1.8, 0)  {$\nu$};
\node (cyzd) at (0, -1.8) {$\xi$};