The idea behind trickle-down economics is that wealth gradually percolates down through the various layers of society and eventually everyone is better off. This theory does not tend to commend itself to those concerned with inequality. First, trickles are by nature far too slow and inadequate for anyone in search of quick solutions. Since inequality is viewed as an 'urgent' problem, solutions that require time to become entrenched are by definition unsatisfactory. Second, trickles of wealth are unpredictable and uncertain - it is difficult to get a clear picture of how matters will unfold and you can't really target the gains in any particular direction.
Justice and equality are excellent ideals to aspire to; since these ideals are offered as the reason why we pay taxes - to collect mandatory tax donations from the rich and use that money to provide resources for the poor so that it's just and equal - nobody can credibly disagree with the need for taxes in a civilised society. The only trouble is, nobody really questions what is really meant by the concepts of Justice and Equality in this context.
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