The fundamental principle is that public or philanthropic capital acts as "catalytic" or "concessional" capital. This means it takes on a disproportionate share of the project's risk or accepts lower returns (or both) to enhance the overall attractiveness for private investors.

Imagine a "capital stack" or a layered structure for a project's financing:

• Bottom Layer (Most Risk, Concessional Terms): This is where public (e.g., from Development Finance Institutions - DFIs, multilateral development banks - MDBs, or government agencies) or philanthropic funds are positioned. They provide first-loss capital (absorbing initial losses), concessional loans (below-market interest rates, longer repayment periods), or grants. Their primary objective is development impact, not maximizing financial return.

• Middle Layer (Mezzanine Finance): This might be provided by impact investors or some DFIs, taking on an intermediate level of risk and return.

• Top Layer (Least Risk, Market-Rate Returns): This is where the commercial private capital comes in (e.g., commercial banks, institutional investors, pension funds). Their objective is to achieve market-rate financial returns, and the de-risking provided by the bottom layer makes the project fall within their acceptable risk-return profile.