[3] During this time he adhered to philosophical positivism and republican democracy, where he frequently joined others is discussions on philosophy and politics, showing a capacity for argument and imagination.

Between many of the causes he defended while a lawyer, in 1890, he was the advocate for António José de Almeida, after he wrote "Bragança, o último" an article against King D. Carlos in the academic journal O Ultimatum.

[citation needed] On 26 August 1876, he joined the Comissão para a Reforma da Instrução Secundária ("Commission on the Reform on Secondary School Instruction").

[citation needed] A member of the Portuguese Republican Party (before the 31 January 1891 revolt), alongside Jacinto Nunes, Azevedo e Silva, Bernardino Pinheiro, Teófilo Braga and Francisco Homem Cristo, he was an active parliamentarian during the constitutional monarchy of King Luís I; he was involved in the debates on the reform of education, the penal code and prisons, in addition to electoral reform.

Following the establishment of the Republic (5 October 1910), young Republican students in Coimbra entered the installations of the Senate, and vandalized the Hall and furniture used in Doctoral ceremonies and damaged paintings of the last Portuguese kings.

[7] As one of the older figures of the Republican regime (he was 71), he was elected President on 24 August 1911; he did not campaign for the position, and noted that it was a heavy burden, which he believed he was personally incapable of fulfilling its duties, but accepted it "for the good of the Republic".

In his personal autobiography, Arriaga recounted how he hoped that he would not be another factor to divide Republicans, especially in a time where there existed a need to work together; it was a difficult period historically, due to the exasperation of the "religious question", constant social agitation and political party instability (associated with "Machiavellian strategies" of some politicians) that fermented during the infancy of the First Republic.

Arriaga deplored the circumstances, going so far as to announcing his intent to resign unless a coalition or non-party government could be installed that resolved the outstanding issues of amnesty and separation of church and state.

[11] But, subsequent governments would not resolve the issue immediately; on 22 February 1914 an amnesty was conceded for those not accused of violent actions, and eleven leaders of subversive groups were released, but the Law of Separation remained unrevised.

But, Joaquim Pereira Pimenta de Castro selected for his ministers, seven military officers, who did not permit the re-opening of Parliament, and provided an amnesty for convicted monarchists involved in the Attack on Chaves[14] He made changes to electoral law and began governing as a dictator, which was only supported by the Evolutionist Party (Portugal) and the group led by Machado dos Santos on the political right of the Republicans.

The parliamentarians, meeting secretly on 4 May 1915 in the Palácio da Mitra, declared Arriaga and Pimenta de Castro outside the law, their acts undemocratic and essentially void.

In his resignation (to his ministers and Party) he defended himself against these unjust accusations and declared his well-intentioned loyalty to the Republican cause, which he had supported throughout his life (but which had abandoned him disillusioned).

[3] His home, near Rua da Janelas Verdes, overlooked the boats in the Tejo, and in the room where he died there were photographs of the two men he most admired, Victor Hugo and Alexandre Herculano, while above his bed, an image of Christ.