The ability to pay doctrine or equal sacrifice principle is vague because it does not have a conclusive definition.
The equal sacrifice principle is based on the assumption that utility from income is cardinally measurable serious objection has been raised against this unrealistic assumption.
Equal sacrifice principle is also based on the unrealistic assumption that marginal utility of income decrease with every increase in income
Serious objection has been raised against the assumption of interpersonal utility consumption under which equal sacrifice principle is valid as people are different physically, mentally or emotionally there can canon utility for every one so that we can compare interpersonal utility in any meaningful sense.
The use of income as an index of ability to pay has serious limitations with this index, the ability to pay theory can’t be extended to cover indirect taxes line those on commodities and services.