At the beginning of January, 2022, amendments to the Income Tax Acts, introduced as part of the project called Polish Deal, entered into force. The basic assumption of the planned changes was to reduce the so-called tax wedge. Income from work is burdened with higher taxes and contributions than income from other sources, especially from economic activity and capital sources. Polish Deal was to reduce these differences by increasing the burden of public levies on income other than from work. However, as a result of subsequent modifications to the draft acts, there was an increase in the burden on income from work and activities performed personally. At the same time, favorable solutions for people choosing a flat-rate tax on recorded income promote the so-called self-employment. This leads to a significant increase in the differentiation of the amount of the tax burden incurred, and thus to the violation of the principle of tax justice.

The tax legislature has a lot of freedom in shaping the tax burden. However, this should not lead to unjustified differentiation of taxpayers' situations, in particular to different taxation of persons in the equal situation. This would result in a breach of the principle of tax justice. In the doctrine of tax law, the 'ability to pay' rule is the basis for the principle of fair taxation. The tax legislature should seek to assess a taxpayer's personal situation in order to assess the tax according to his/her ability to pay.

The aim of this research is to analyze how the theoretical assumption of fair taxation is accomplished in the personal income tax act. Research scope includes rules of distinguish tax revenue sources, then differentiation income taxation. The analysis showed that the 'ability to pay' rule is not taken into account by the legislator in the process of shaping income, and thus the individual tax burden. This leads to unfair taxation in a horizontal level (horizontal tax justice).