Direct income inequalities from trade are inevitable in any market. Incomes cannot be distributed evenly in the first instance because we do not spend evenly. When we trade it is always the case that some products (ideas) are more popular than others, and as soon as money becomes economically necessary some of us accumulate more wealth than we can consume immediately, so wealth imbalances occur.
Free and open trade with a common currency (or easily convertible common currencies) is the only efficient mechanism for determining what is needed and distributing limited resources in our complex society. Central control cannot supplant free trade, as production and distribution decisions are too complex for this. Attempting to do so limits diversity, freedom and growth.
Because direct income equality from free trade is impossible, and free trade is vital, a shared base income (SBI), ensuring equal opportunity for all to realise potential, must be created by sharing the earnings we make on our trade (the earnings we make from sale of our products and services after our expenses involved in producing them).
Shared earnings and a shared base income do not eliminate income inequality, some of us still earn incomes far greater than the base, but sharing half of all the income we earn from trade, via a base income shared between us, helps eliminate poverty and ensures we all have the opportunity to realise our potential. It also inculcates a common spirit of sharing in our society, a sense of community and belonging.
Why half? Because it is an equal share, half for ourselves, half for others. Such a share is also simple - simple to comprehend and simpler to implement than any tiered system. It is also more agreeable because it is transparently fairer.
[Excerpt from The Common Purpose Manifesto]